point-based ionizing radiation dosimetry using radiochromic materials and … · 2013. 11. 8. ·...

POINT-BASED IONIZING RADIATION DOSIMETRY USING RADIOCHROMIC

MATERIALS AND A FIBREOPTIC READOUT SYSTEM

by

Alexandra Rink

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Graduate Department of Medical Biophysics University of Toronto

© Copyright by Alexandra Rink (2008)

ii

Abstract

Point-Based Ionizing Radiation Dosimetry Using Radiochromic Materials And Fibreoptic

Readout System

Doctor of Philosophy, 2008

Alexandra Rink

Department of Medical Biophysics

University of Toronto

Real-time feedback of absorbed dose at a point within a patient can help with radiological

quality assurance and innovation. Two radiochromic materials from GafChromic MD-55 and

EBT films have been investigated for applicability in real-time in vivo dosimetry of ionizing

radiation. Both films were able to produce a real-time measurement of optical density from a

small volume, allowing positioning onto a tip of an optical fibre in the future. The increase in

optical density was linear with absorbed dose for MD-55, and non-linear for EBT. The non-

linearity of EBT is associated with its increased sensitivity to ionizing radiation compared to

MD-55, thus reaching optical saturation at a much lower dose. The radiochromic material in

EBT film was also shown to polymerize and stabilize faster, decreasing dose rate dependence in

real-time measurements in comparison to MD-55. The response of the two media was tested

over 75 kVp – 18 MV range of x-ray beams. The optical density measured for EBT was constant

within 3% throughout the entire range, while MD-55 exhibited a nearly 40% decrease at low

energies. Both materials were also shown to be temperature sensitive, with the change in optical

density generally decreasing when the temperature increased from ~22°C to ~37°C. This was

accompanied by a shift in the peak absorbance wavelength. It was illustrated that some of this

decrease can be corrected for by tracking the peak position and then multiplying the optical

density by a correction factor based on the predicted temperature. Overall, the radiochromic

material in GafChromic EBT film was found to be a better candidate for in vivo real-time

dosimetry than the material in GafChromic MD-55.

A novel mathematical model was proposed linking absorbance to physical parameters

and processes of the radiochromic materials. The absorbance at every wavelength in the

spectrum was represented as a sum of absorbances from multiple absorbers, where absorbance is

characterized by its absorption coefficient, initiation constant, and polymerization constant.

Preliminary fits of this model to experimental data assuming two absorbers suggested that there

is a trade-off between EBT’s greater sensitivity and its dose linearity characteristics. This was

confirmed by experimental results.

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Dedicated to my dear parents.

Acknowledgments The assistance and wisdom of many people went into this work. I would gratefully like to

acknowledge the contributions, in whatever form they were, of the following:

• my co-supervisors, Dr. David Jaffray and Dr. Alex Vitkin, for all the paper edits, research

guidance and advice

• committee members, Dr. Christine Allen and Dr. Mike Rauth for all the support

• Yuen Wong, Brian Taylor, Jason Ellis, and Matt Filletti for machining all the phantoms

and doing the various small “rush” jobs

• Robert Rothwell and Robert Rusnov for all the assistance with electrical and optical work

• Dr. Robert Heaton, Hamideh Alasti, Duncan Galbraith, Dr. Mohammad Islam, and Dr.

Jean-Pierre Bissonnette for their experience

• Bern Norrlinger for his experience and help with any and every accelerator that ever

broke

• Tony Manfredi for all the assistance with the Elekta accelerators

• Dr. Robert Weersink and David Giewercer for assistance and wisdom with fibre optics

• Joanne Kniaz of Advanced Optical Microscopy Facility for the microscopy work

• Dr. David Lewis and Dr. Sangya Varma of International Specialty Products for their

contributions to this work, experience and guidance

• Dr. Douglas Moseley for all the help with Matlab, the carpool rides, and outrageous

conversations on the GO train

• Steve Ansell and Graham Wilson for all the computer support, psychotherapy lunches

and Chinese noodles

• Jinzi Zheng and Jeremy Hoisak for all the coffee breaks which kept me sane

• my parents, Gala and Youri Rink, for never looking back or regretting any choices in life

v

Table of Contents CHAPTER 1: INTRODUCTION.................................................................................................. 1

I. Ionizing Radiation in Cancer Treatment ............................................................................... 2

II. Radiation Dose ...................................................................................................................... 3

III. Radiation Dosimetry ............................................................................................................ 5

A. Basic Interactions.............................................................................................................. 5

B. Standards and Protocols for Dosimetry............................................................................ 6

C. Estimation of Dose Delivered in Therapy......................................................................... 7

IV. The Challenges of Dose Measurement in the Clinical Setting ............................................ 7

A. Clinical Applications and Ideal Dosimeter ....................................................................... 7

B. Current in vivo Dosimeters.............................................................................................. 10

C. Optical Methods .............................................................................................................. 11

V. Outline of Thesis................................................................................................................. 13

CHAPTER 2: REAL-TIME RESPONSE OF GAFCHROMIC® MD-55 FILM TO IONIZING

RADIATION ................................................................................................................................ 21

I. Introduction ......................................................................................................................... 22

Review of GafChromic® MD-55 .......................................................................................... 22

General Experience............................................................................................................... 22

Solid-state Polymerization of Diacetylenes .......................................................................... 25

II. Methods and Materials ........................................................................................................ 29

A. ΔOD of GafChromic® MD-55 at Various Doses............................................................ 38

B. Sensitivity as a Function of Layer Thickness.................................................................. 39

C. ΔOD of GafChromic® MD-55 at Various Dose Rates.................................................... 39

D. ΔOD Dependency on Temperature ................................................................................. 40

E. Continuous Versus Pulsed Irradiation ............................................................................. 40

III. Results................................................................................................................................ 41

A. ΔOD of GafChromic® MD-55 at Various Doses............................................................ 41




E. Continuous Versus Pulsed Irradiation ............................................................................. 49

IV. Discussion.......................................................................................................................... 49

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A. OD of GafChromic® MD-55 at Various Doses .............................................................. 50




E. Applications..................................................................................................................... 54

V. Conclusion .......................................................................................................................... 55

ACKNOWLEDGEMENTS.................................................................................................. 56

CHAPTER 3: REAL-TIME RESPONSE OF GAFCHROMIC® EBT ....................................... 61

I. Introduction ......................................................................................................................... 62

II. Method and Materials.......................................................................................................... 62

A. ΔOD of EBT Film Versus Time ..................................................................................... 66

B. Sensitivity and Stability Comparison Between EBT and MD-55 Films......................... 66

C. Dependence of Real-Time OD Measurements on Dose Rate for the EBT Film ............ 67

D. Structure of Active Crystals in MD-55 and EBT Films.................................................. 67

III. Results and Discussion ...................................................................................................... 67

A. OD of EBT Film Versus Time........................................................................................ 67

B. Sensitivity and Stability Comparison Between EBT and MD-55 Films......................... 70

C. Dependence of Real-Time ΔOD Measurements on Dose Rate for the EBT Film.......... 73

D. Structure of Active Crystals in MD-55 and EBT Films.................................................. 75

IV. Conclusion ......................................................................................................................... 77


CHAPTER 4: EFFECTS OF VARYING DOSE RATE ON REAL-TIME MEASUREMENTS

OF OPTICAL DENSITY OF GAFCHROMIC® EBT ................................................................. 81

I. Introduction ......................................................................................................................... 82

II. Methods and Materials ........................................................................................................ 82


IV. Conclusion ......................................................................................................................... 88


CHAPTER 5: CHARACTERIZATION OF GAFCHROMIC® EBT: TEMPERATURE AND

HUMIDITY EFFECTS................................................................................................................. 91

I. Introduction ......................................................................................................................... 92

Chemical Background and General Experience ................................................................... 92

II. Methods and Materials ........................................................................................................ 94 vii

A. Temperature Dependence ................................................................................................ 96

B. Absorbance and Sensitivity Dependence on Water Content............................................ 97


A. Temperature Dependence ................................................................................................ 99

B. Absorbance and Sensitivity Dependence on Water Content.......................................... 104

IV. Conclusion ....................................................................................................................... 109

ACKNOWLEDGEMENTS................................................................................................ 109

CHAPTER 6: ENERGY DEPENDENCE OF GAFCHROMIC® ............................................. 112

MD-55 AND EBT....................................................................................................................... 112

I. Introduction ....................................................................................................................... 113

II. Methods and Materials ...................................................................................................... 114

Solid Water™ Phantom ...................................................................................................... 114

Ionizing Radiation Exposures ............................................................................................. 115

Optical Measurements ........................................................................................................ 117

III. Results and Discussion .................................................................................................... 118

IV. Conclusion ....................................................................................................................... 123


CHAPTER 7: MATHEMATICAL MODEL OF RADIOCHROMIC MEDIUM RESPONSE TO

IONIZING RADIATION ........................................................................................................... 127

I. Introduction ....................................................................................................................... 128

II. Methods and Materials ...................................................................................................... 129

III. Results and Discussion .................................................................................................... 132

V. Conclusion ........................................................................................................................ 138


CHAPTER 8: SUMMARY AND FUTURE DIRECTIONS..................................................... 142

I. Summary............................................................................................................................. 143

II. Future Directions............................................................................................................... 146

A. Optical Probe................................................................................................................. 146

B. Organization of Monomers and Polymers .................................................................... 148

C. Importance of Chemical Composition and Structure.................................................... 148

D. New Radiochromic Materials ..................................................................................... 149

E. Polymerization Kinetics as a Function of Dose Per Pulse.......................................... 149

F. ............................................................................ 150 Model-Fitting Algorithm and Code

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List of Tables Table 1. List of criteria for in vivo point-based real-time dosimeter. ............................................ 9

Table 2. Evaluation criteria for in vivo point-based real-time dosimeter. ................................... 23

Table 3. Comparison of inferred dose and percent error using calibration plot and pre-exposure

calibration as methods of calculation............................................................................................ 44

Table 4. Coefficients of equations of best fit characterizing ΔOD/DGy as a function of dose rate.

....................................................................................................................................................... 87

Table 5. Average percent standard deviation for each dose, uncertainty, and the difference

between the two for each dose delivered. ..................................................................................... 88

Table 6. Correction factors for the temperature correction scheme, calculated for doses of 50 –

400 cGy shown for a selection of predicted temperature values.. .............................................. 103

Table 7. The x-ray and photon beams employed in the investigations...................................... 116

Table 8. Comparison of response of EBT film, normalized to response at 6 MV, as measured

approximately 24 hours after exposure to that measured immediately at the end of exposure .. 122

Table 9. Model parameters using two absorbers for MD-55 and EBT and a fit with 1 second

pulse averaging. .......................................................................................................................... 136

List of Figures Figure 1. Schematic of a typical relationship between tumour control probability (TCP) and

normal tissue complication probability (NTCP) versus dose. ........................................................ 4

Figure 2. Structures of: (a) diacetylene monomers, upon exposure to ionizing radiation,

polymerizes into (b) butatriene structure polymer; as the polymer chain grows, it rearranges via

(c) an intermediate between butatriene structure and acetylene structure, into (d) acetylene

structure polymer. ......................................................................................................................... 26

Figure 3. A model of optical density of GafChromic® MD-55 versus time before, during and

after exposure................................................................................................................................ 28

Figure 4. Schematic of experimental setup.................................................................................. 30

Figure 5. Emission spectrum of the LED as detected by the spectrophotometer. ....................... 31

Figure 6(a-c). Solid Water™ phantom (a) assembled, (b) disassembled, (c) schematic. ............ 32

Figure 7. Schematic of cross-section of film holder in Solid Water™ phantom.. ....................... 33

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Figure 8. Change in absorbance of GafChromic® MD-55 film at various wavelengths plotted

before exposure, immediately after end of exposure, and 15 and 60 minutes after the end of

exposure. ....................................................................................................................................... 34

Figure 9. Change in optical density and rate of change in optical density for GafChromic® MD-

55 film as a function of time before, during and after exposure to 381 cGy with 6 MV X-rays.. 35

Figure 10. Termination of exposure is taken as the intercept of the two fitted lines: first line

corresponding to data obtained during exposure and second line corresponding to data obtained

after end of exposure..................................................................................................................... 36

Figure 11. Change in optical density can be calculated for any exposure by subtracting initial

OD from final OD......................................................................................................................... 37

Figure 12. Schematic of setup for temperature dependency experiments. .................................. 40

Figure 13. Change in optical density for GafChromic® MD-55 exposed to 381 cGy with 6 MV

X-rays as a function of time.......................................................................................................... 41

Figure 14. Change in OD for five pieces of film, each exposed to 381 cGy with 6 MV X-rays (at

the doserate of 285 cGy/min)........................................................................................................ 42

Figure 15. Inferred dose using ΔOD measurements and calibration plot as a function of applied

dose. .............................................................................................................................................. 42

Figure 16. Change in optical density for a piece of GafChromic® MD-55 film during several

exposures applied approximately 5 minutes apart. ....................................................................... 43

Figure 17. Optical density as a function of dose for a system utilizing one, two and four pieces

of stacked film............................................................................................................................... 45

Figure 18. Change in optical density as a function of dose for doses delivered at 95 cGy/min,

286 cGy/min and 671 cGy/min..................................................................................................... 46

Figure 19. Rate of change in optical density as given by the linear fit of data obtained during

exposure as a function of applied dose, for doses delivered at 95 cGy/min, 286 cGy/min and 571

cGy/min......................................................................................................................................... 47

Figure 20. Position of wavelength of maximum absorbance for GafChromic® MD-55 as a

function of irradiation/measurement temperature......................................................................... 48

Figure 21. Change in OD for a given dose as a function of applied/measured temperature using

both a constant spectral averaging window and a shifting spectral averaging window. .............. 48

Figure 22. Schematic of experimental setup................................................................................ 63

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Figure 23. Emission of the light emitting diode used in experimental setup, as measured by

spectrometer.................................................................................................................................. 63

Figure 24. Schematic of layers in EBT film. ............................................................................... 64

Figure 25. Change in absorbance of EBT film over a range of wavelengths before, immediately

after, and at two time points post-exposure. ................................................................................. 65

Figure 26. Optical density versus time for a single piece of EBT film; optical density versus time

for five pieces of EBT film shown on a reduced time scale (inset). ............................................. 68

Figure 27. Wavelength of maximum absorbance for EBT film versus time during and after

exposure to 9.52 Gy at 2.86 Gy/min with 6 MV X-rays............................................................... 69

Figure 28. Optical denisty of EBT film versus time for various spectral averaging windows.... 70

Figure 29. Optical density for EBT and MD-55 films during and after exposure....................... 71

Figure 30. Percent increase in OD for EBT and MD-55 films after exposure, calculated with

respect to OD at the end of exposure. ........................................................................................... 72

Figure 31. Percent increase in OD for EBT and MD-55 films within one hour after exposure. . 73

Figure 32. Optical density for EBT film exposed to 9.52 Gy, delivered with 6 MV at 0.95

Gy/min and 5.71 Gy/min. ............................................................................................................. 74

Figure 33. Microscope images of monomer crystals within the sensitive media of MD-55 and

EBT films...................................................................................................................................... 76

Figure 34. Optical density versus time for a 50 cGy irradiation at 16 cGy/min.......................... 84

Figure 35. The average sensitivity as a function of dose rate, for various doses......................... 85

Figure 36. Chemical formula of pentacoasa-10,12-dyinoic acid (PCDA), and lithium salt of

PCDA (LiPCDA).. ........................................................................................................................ 93

Figure 37. Change in absorbance spectra for EBT film exposed to 1 Gy with 6 MV and 75 kVp

beams.. .......................................................................................................................................... 95

Figure 38. Schematic of the modified phantom, with plastic water hoses on either side of the

film and optical fibers. .................................................................................................................. 96

Figure 39. Wavelength of maximum change in absorbance of commercial EBT films irradiated

to 1 Gy as a function of measured temperature. ......................................................................... 100

Figure 40. Values of wavelength of maximum absorbance for various doses delivered to

commercial EBT films as a function of measured temperature.................................................. 100

Figure 41. Change in optical density for 1 Gy dose calculated for optical range of 630-640 nm,

and an optical range of 10 nm centered about wavelength of maximum absrobance, versus

measured temperature.. ............................................................................................................... 101

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Figure 42. Temperature calculated using the position of wavelength of maximum absorbance

versus measured temperature, shown with a line of best fit.. ..................................................... 102

Figure 43. Change in optical density for films irradiated to 1 Gy using a fixed optical integration

range of 630 to 640 nm, moving optical range of 10 nm about the peak of maximum absorbance,

and as calculated using the peak of maximum absorbance and temperature-dependent correction

factor.. ......................................................................................................................................... 102

Figure 44. Percent decrease in net OD for a 3 Gy dose, following different times in a desiccator

at 50 ºC........................................................................................................................................ 105

Figure 45. Spectral comparisons of absorbance of desiccated and normal unlaminated EBT film.

..................................................................................................................................................... 106

Figure 46. Absorbance of unlaminated EBT film after time in desiccator at 50 ºC. ................. 107

Figure 47. Spectral comparisons of absorbance of desiccated, rehydrated, and normal

unlaminated EBT film irradiated to 3 Gy. .................................................................................. 107

Figure 48. Absorbance spectra of exposed unlaminated films using “plate-like” form of

polymer, and the rehydrated form of “hair-like” polymer. ......................................................... 108

Figure 49. 30 cm × 30 cm × 4 cm phantom with the film insert ............................................... 114

Figure 50. Sample of time-dependent OD with time for a 1 Gy irradiation with a 75 kVp

Therapax DXT 300 beam at 8 cGy/min for MD-55, HS, and EBT film. ................................... 119

Figure 51. Un-normalized change in OD for 1 Gy total dose for MD-55, HS and EBT films for

irradiations delivered at various equivalent x-ray energies ........................................................ 119

Figure 52. Change in OD per Gy for MD-55, HS and EBT, as a function of equivalent x-ray

energy.......................................................................................................................................... 120

Figure 53. Increased sensitivity of HS and EBT films with respect to MD-55, for a dose of 1 Gy

..................................................................................................................................................... 123

Figure 54. Change in absorbance of a single absorber versus time for different A parameters,

keeping k and p parameters constant. ......................................................................................... 132

Figure 55. Change in absorbance of a single absorber versus time for different k parameters,

keeping A and p parameters constant.......................................................................................... 133

Figure 56. Change in absorbance of a single absorber versus time for different p parameters,

keeping A and k parameters constant. ......................................................................................... 134

Figure 57. Experimental absorbance of MD-55 film at the main absorbance peak, and the model

fit.. ............................................................................................................................................... 135

xii

Figure 58. Experimental absorbance of EBT film at the main absorbance peak, and the model

fit.. ............................................................................................................................................... 135

Figure 59. Schematics of single and dual fibre optical dosimeter prototypes. .......................... 147

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List of Abbreviations and Symbols ΔA change in absorbance

ΔOD change in optical density

ΔODv change in visual density

ΔV small volume

ε(λ) extinction coefficient at wavelength λ

λ wavelength

λmax wavelength of maximum absorbance

ρ density 60Co Cobalt-60

A absorbance

A(λ) absorbance at wavelength λ

AAPM American Association of Physicists in Medicine

bi polymer initiation constant per dose

c concentration

cGy centiGray

D dose

DGy dose in Gy

EBT radiochromic film intended for External Beam Therapy

E energy

eV electron-volt

FWHM full width half maximum

Gy gray (J/kg)

HS radiochromic film of High Sensitivity

HVL half-value layer

I intensity

ID background intensity

IR reference intensity

I0s initial intensity

Is sample intensity

ICRU International Commission of Radiation Units

IMRT intensity modulated radiation therapy

IGRT image guided radiation therapy

ISP International Specialty Products

ki polymer initiation constant

keV kilo electron-volt

kVp peak kilovoltage

l length

LED light emitting diode

LINAC linear accelerator

LiPCDA Lithium pentacosa-10,12-diynoate (lithium salt of PCDA)

MD-55 radiochromic film intended for Medium Dose, size 5×5

MeV mega electron-volt

MOSFET metal oxide semiconductor field-effect transistor

MSDS Material Safety Data Sheet

MV megavolt

N0 initial number of monomer chains

Nm remaining number of monomer chains

Nip number of initiated polymer chains

Nfp number of fully-formed polymer chains

NTCP normal tissue complications probability

OD optical density

ODv visual density (weighted by known response of human eye)

OSL optically stimulated luminescence

pi polymerization kinetics constant

PCDA pentacosa-12,12-diynoic acid

PDD percent depth dose

PMMA poly-methyl methacralate

QTH quartz-tungsten-halogen

SAD source-to-axis distance

SSD source-to-surface distance

t time

TCP tumour control probability

TG Task Group

TLD thermoluminescent dosimeter

xv

TPR tissue-phantom ratio

UV ultraviolet

Z atomic number

xvi

1

CHAPTER 1: INTRODUCTION

2The work within this thesis describes a novel method for performing real-time dosimetry using

fibre-optic read-out of radiochromic materials. The radiochromic materials are investigated for

their applicability in clinical dosimetry measurements in vivo and in vitro. Their performance as

a function of dose and time is modeled, with parameters linked to physical properties of the

materials and the processes that occur during exposure to radiation. A system is thus established

for evaluating radiochromic dosimeters for clinical dosimetry, whereby their performance can be

at least in part be predicted by their physical properties.

In this chapter, clinical rationale for the proposed real-time dosimeter is established by

outlining the need for in vivo dosimetry and the inability of the dosimeters presently available on

the market to meet that need.

I. Ionizing Radiation in Cancer Treatment

Ionizing radiation is encountered under many circumstances in medicine, and specifically in

oncology. It is used to identify and locate the cancer, to target it, and to treat it, with

approximately 50% of cancer patients receiving radiation therapy for management of their

disease.* High energy photons (referred to as x-rays and gamma rays) are known to damage

tissue. Although the exact details of tissue damage are still not fully understood, it is believed

high energy photons induce ionization of important molecules within the cells, such as

deoxyribonucleic acid. Ionization refers to removal of an electron from a molecule, making it

unstable. These unstable molecules may react in a way that would prevent them from

functioning properly, eventually leading to cell death.

The source of radiation can be external or internal (known as brachytherapy), varying

greatly in energy and intensity. Energy can vary from 21 – 660 keV(1) gamma rays from

decaying radionuclides in brachytherapy seeds, to 18 MV x-ray or 20 MeV electron beams from

a linear accelerator (LINAC). The dose rate can be as low as 0.4-2 Gy/h(1) (where Gy=1J/kg) for

low dose rate brachytherapy, to as high as 6 Gy/min for LINAC treatments. The majority of the

external treatments are divided into dose fractions delivered daily, five days a week, over several

weeks, with only a small percentage of treatments delivered in a single large dose (known as

stereotactic radiosurgery) from a 60Co unit called GammaKnife or from a LINAC using a 6 MV

x-ray beam. In recent years, treatments have become more conformal to the tumour due to

implementation of Intensity Modulated Radiation Therapy (IMRT) and Image-Guided Radiation

Therapy (IGRT). With the development of these new technologies, a trend has been evolving * http://www.cancer.gov/cancertopics/factsheet/Therapy/radiation

3towards higher target absorbed dose values, fewer fractions, smaller treatment volumes, and

steeper dose gradients. These developments need to be validated in terms of actual dose

delivered.

II. Radiation Dose

Cell damage from ionizing radiation may result in several different outcomes, including repair of

damage by the cell, cell death, and survival with mutation.(2) Depending on the type of damage

and the tissue irradiated, the biological effect can take anywhere between a few hours (acute) to

many years (late) to manifest. During radiation therapy, the dose and its distribution are

important for the outcome of the treatment and prevention of further complications. A high

enough dose has to be delivered to the tumour and affected organs to obtain high probability of

tumour control, and a minimal dose should be delivered to healthy surrounding organs to limit

probability of acute or late effects.(2) To maintain high probability of tumour control, the

International Commission of Radiation Units (ICRU) recommends uniformity of tumour dose

within +7/-5% of the total prescribed dose.(3) The upper limit exists because the dose prescribed

is often limited by the dose delivered to the surrounding healthy organs during irradiation, which

is dependent on the type of treatment delivery. Generally, conformal treatment allows for higher

dose to be delivered to the tumour and for lower dose be delivered to the surrounding tissues,

though the total volume of tissue irradiated may increase. On the other hand, the probability of

cancer recurrence due to geometric miss may increase.

The probability of biological effect taking place (whether it be tumour cell kill or normal

tissue complications) versus dose is called a dose response curve.(2) The curves for tumour

control probability (TCP) and normal tissue complication probability (NTCP) are often plotted as

sigmoid relationships (Figure 1). That is, there is nearly no effect at first, then the probability

rises sharply, and levels off to a plateau. Thus delivering a smaller dose to the tumour than that

required for cure or control would sharply increase the probability of relapse. On the other hand,

if the patient receives a dose to the tumour that is much higher than that prescribed, then it is also

likely that the patient receives a higher dose to the surrounding normal tissue. Given the

sigmoidal relationship between dose and effect, the risk of acute or late effects increases

dramatically.

Because of the conformality of modern treatments, describing the dose distribution by a

single dose to the tumour and using the +7/-5% recommendation for guidance is an

oversimplification. As the treatments become tailored to each patient’s needs, the dose

41.0

0.5

Prob

abili

ty

Dose (Gy)

TCP NTCP

0.9

+5%-5%

30 40 50

Figure 1. Schematic of a typical relationship between tumour control probability (TCP) and

normal tissue complication probability (NTCP) versus dose. The NTCP curve is based on two

values (marked by X): doses at which 5% and 50% of patients develop complications when 2/3

of their liver is irradiated.(4) Dashed line represents a dose of 42.5 Gy, yielding 90% probability

of tumour control, and 37.5% probability of normal tissue complications (liver failure).

Increasing the tumour dose by 5% (dotted line) of the prescribed dose increases TCP by only

2.5%, but increases NTCP to 50%. On the other hand, delivering the same distribution with 5%

lower dose (dotted line), decreases NTCP to 25%, but also decreases TCP to 82.5%, which may

compromise treatment outcome.

distributions become more diverse. Systematic errors larger than 5% in dose delivery (measured

as entrance, exit dose, or combination of the two during treatment) to a small percent of patients

(~ 1%) have been published.(5-7) These can be due to inadequacies in dose calculation

algorithms,(8) setup errors, or a human error on behalf of the many individuals involved in the

process of patient treatment. Although doses to the patient can be calculated or inferred from a

5relative measurement, the American Association of Physicists in Medicine Task Group 40

recommended that clinics have access to an in vivo dosimetry system.(9) While radiation

transport algorithms are constantly being improved in order to perform dose calculations, and

phantoms become more complex to better represent human anatomy, conditions where dosimetry

and simulation of humans remain a challenge still exist. Accurate assessment of the dose

distribution in brachytherapy treatment and in regions near in homogeneities for external beam

treatments is vital to rapid innovation and development of new radiation therapy technologies

and techniques. Such an assessment can be done by performing in vivo dose measurements.

However, performing such measurements under clinical conditions is challenging.

III. Radiation Dosimetry

A. Basic Interactions

The interaction of ionizing radiation with matter results in a dose deposited within that

matter, where dose is defined as the absorbed energy per mass (J/kg, or Gy).(10) In radiotherapy,

the dose quoted is often dose to water, as most tissues within the body have similar radiological

properties as water (common exceptions are lung tissue, bone and teeth). The ionizing radiation

can be directly ionizing (charged particles) or indirectly ionizing (photons).(10)

The photons, as they pass through the medium, are attenuated by the medium and scatter

from their original path. The processes of attenuation are due to coherent scattering,

photoelectric effect, Compton effect, pair/triplet production, or photonuclear interactions. In

radiation therapy, the middle three are the interactions important for dose deposition, and the

probability of any of these events happening when a photon beam passes through the medium

depends on the energy of the photon, density, and the atomic number (Z) of the medium.

Photoelectric effect is the predominant interaction for low-energy photons (below 100 keV). The

probability of photon interacting in this manner in a medium of a given density increases with Z3

of the medium. Here all of the photon’s energy is transferred to one of the inner shell electrons

of an atom. This electron then continues through the medium with a kinetic energy equal to the

energy of the initial photon less the binding energy of the electron. In a Compton interaction, the

photon interacts with a valence shell electron, transferring part of photon’s energy to an electron

and the two then continue at a given angle from each other from the point of interaction, such

that the momentum is conserved. In this interaction, the photon does not transfer all of its energy

to the electron. The probability of this interaction occurring in a medium of given density is

6approximately independent of Z of the medium, and is predominant for photon energies around 1

mega-electron-volt (MeV). For high energy photons (over 1.022 MeV) pair production can

occur. In these interactions, as the photon interacts with the Coulomb field near the nucleus, the

photon is absorbed giving rise to an electron and a positron. If the photon interacts with the field

of the atomic electron, the atomic electron also acquires energy and escapes from the atom, thus

yielding triplet production. The process of triplet production requires the energy of the photon to

be greater than 2.044 MeV. The probability of pair production for a medium of given density is

roughly proportional to Z, while triplet production is independent of Z of the medium.(10)

The electrons that result from these interactions of photons with the medium in turn travel

through the medium themselves. The electron interacts with almost every atom in its path whose

electric field is detectable. Most of these interactions transfer small fractions of the electron’s

energy, and thus the electron is often thought of as losing the energy gradually in a frictionlike

process. Some of these energy losses result in a photon (Bremsstrahlung) being emitted when

the electron changes direction due to an electric field from a nearby nucleus. The energy lost in

this way does not contribute to the locally deposited dose. Electron interactions that do

contribute to local dose will either excite the shell electrons of a nearby atom to a higher energy

level, or ionize it. The rate of energy loss per distance decreases with increasing Z of the

medium and increasing kinetic energy of the electron.(10)

B. Standards and Protocols for Dosimetry

Ionizing radiation dose can be measured well in standard conditions following set

protocols(11-14). In most cases, using dosimetry equipment calibrated at a national standard

laboratory (such as National Research Council of Canada) or traceable to such a calibration, and

performing measurements under controlled conditions allows for accurate dosimetry in air and in

water or plastic phantoms, with uncertainties below 1%.(12,15) However, in some cases, such as

intravascular brachytherapy, absorbed dose standards may vary by as much as 10% between

measurements and different Monte Carlo calculations.(16) When using the dosimetric gold

standard, the ion chamber, some of the controlled conditions include a known temperature and

pressure, type and energy of the beam, distance from the source and depth (if phantom is

used).(12) Unfortunately, these parameters may not always be known to a high level of precision

and accuracy in clinical conditions.

7C. Estimation of Dose Delivered in Therapy

Doses under uncontrolled conditions, delivered during imaging and therapy, can be

obtained in several different ways. They can be computed using various algorithms as is done in

actual treatment planning,(17) with the algorithms constantly improving to include dose

calculations for brachytherapy and imaging procedures, as well as external beam irradiations.(18-

21) The dose can also be inferred from a relative dose measurement performed during the

procedure (such as skin dose measurement), or measured directly at a point of interest. Because

the points of interest on the patient may be inside the patient, such as at the tumour site,

performing a direct measurement is often trickier than the other two approaches.

IV. The Challenges of Dose Measurement in the Clinical Setting

While real-time dosimetry may be useful for in vitro measurements at several points of time

varying radiation fields, such as those in high dose rate brachytherapy or IMRT, the discussion

of clinical applications here is kept to in vivo measurements. Predicting the three-dimensional

cumulative dose distribution within a patient over the course of their treatment can be complex,

given the variations arising from minor patient positioning errors, treatment beam fluctuations,

motion during treatment, changes in anatomy as the patient loses weight or the tumour shrinks,

and possibly other sources of error. Thus measurements of dose at or within a patient are often

desired as part of a quality assurance, for investigative purposes of a new procedure, or for

implementation of new technology protocol. Performing dose measurements inside patients is

more complicated than skin dose measurements, and is not nearly as straightforward as phantom

dosimetry which is often utilized in the clinic.

A. Clinical Applications and Ideal Dosimeter

A dosimeter that can accurately measure ionizing radiation dose in vivo under various

clinical conditions may simplify and improve the current state of dosimetry. It can be used for

quality assurance of both external beam and brachytherapy treatments. If an error in positioning,

machine output, transcription or some other error occurs that results in a discrepancy of planned

dose above a set threshold, treatment can be interrupted, and the discrepancy investigated. The

dosimeter may also be used to track dose in an organ that moves in and out of the radiation field

during respiration. In a brachytherapy procedure, the real-time dose rate measurement may be

used as feedback to verify proper positioning of seeds. If the dose rate is higher or lower than

8anticipated, the insertion of the next set of seeds may be adjusted to get the proper dose rate and

dose at the point of measurement.

For a dosimeter to be an appropriate option for in vivo measurements in most clinical

scenarios, the overall construction and the dosimetric medium must satisfy several requirements,

listed in Table 1. The requirements can be used as a set of guidelines for evaluation of a new

dosimetric medium. The presence of the dosimeter must not perturb the tissue or the dose

distribution. It must also be sufficiently small (1) to be able to resolve a sharp dose gradient in a

radiation field. However, the size of the sensitive medium must be sufficiently large to yield

good signal statistics for high precision measurements. In part, the signal statistics can be

controlled if the dosimeter uses passive read-out method, as is discussed later in this Chapter.

The dosimeter should have near water-equivalent composition (2), such that its response

with change in energy is similar as that of water.(22) This allows for a single dosimeter to be used

across all energies encountered both in external beam and brachytherapy treatments without

performing a separate calibration. The dosimeter for quality assurance purposes should respond

in real time (3), providing dose or dose rate estimate within a few seconds from beginning of

irradiation so that treatment can be interrupted if the dose rate is outside of the value expected.

The dosimeter also has to show a dose response that is independent of the dose rate used to

deliver that dose (6). The dose estimate should have high dose resolution (5) in order to detect

small variations (~1 cGy) in the dose delivered (daily fractions are often ~200 cGy, and total

doses are 24-70 Gy, depending on the treatment and fractionation pattern), and the response

should be ideally linear with dose (4) for simple computation. Some of the other issues to be

taken into consideration are environmental conditions (7), such as the temperature dependence of

the dosimeter. The dose estimate should be independent of the temperature of the dosimeter,

which can vary from near room temperature on skin surface to as high as ~38 °C within the

patient. Finally, the dosimeter must be non-toxic (8) and biocompatible, and would preferably

be inexpensive enough to be disposable after each patient.

A dosimeter, as described above, which can be used across a wide range of energies

could be implemented in both external beam radiation therapy and brachytherapy, simplifying

the dosimetric procedures to a single device. Accurate assessment of the dose given to the

patient using a reliable dosimeter with a water-equivalent response would save time and money

over using multiple dosimeters with a significant over- or under-response to low energy X-rays

and inferring the dose via correction factors and calculations.

9Table 1. List of criteria for in vivo point-based real-time dosimeter.

Criterion

#

Criterion Comments

1 small size(22)

(

10B. Current in vivo Dosimeters

Several dosimeters currently available on the market are used for some in vivo

dosimetry measurements. These are in vivo ion chambers, diodes and MOSFETs (metal oxide

semiconductor field effect transistors). An ion chamber is classified as an absolute dosimeter: it

can be used to measure the absorbed dose to its own sensitive volume without any calibration.(10)

However, it often needs to be calibrated as knowing the exact measurement volume, and mass of

air contained, is required for absolute dosimetry. The ion chamber measures the total ionization

produced by electron interactions in air, the charge is collected by an electrode set at a high

voltage, and this value can be related to dose. The mass of air contained must yield reasonable

signal statistics, limiting in vivo ion chambers to dosimetry in bladder and rectum, too large for

use in tissue.

A silicon semiconductor diode is what is generally used as an in vivo radiation detector.

When the diode is exposed to ionizing radiation, electron-hole pairs are created throughout the

diode, and as they move through the diode, a measurable current is created. The amount of

silicon required for measurement (known as the die) is 0.01-0.1 mm3, and the current related to

deposited dose can be measured in real-time via the coaxial cable by the electrometer.(23) For in

vivo dosimetry, the die is covered with material for protection and for proper buildup. The

buildup is necessary for skin dose measurements to give the photons enough medium to interact

with in order to create the electrons that will in turn interact with the silicon.(23) This buildup

and the protective cover make the diodes rather bulky (up to 3 cm in length and ~7 mm in

diameter), making it difficult or impossible to position into all tissues of the patient because of

their size. Some diode configurations also suffer from angle dependence of their response (due

to inhomogeneity of the buildup material and coaxial cable attached), and they have been shown

to perturb the dose distribution directly behind the diode by as much as 30%, with the effect

more pronounced at lower beam energies.(24) Read-out was also shown to vary with

temperature,(24,25) complicating dosimetry further by the fact that the temperature of the diode

may not always be known during its use.

A MOSFET is a small silicon transistor.(26) A p-type silicon semiconductor sits on an

insulating oxide layer, which separates it from a conducting metal band.(27) The p-type silicon

has positive charges accumulated within, proportional to the negative voltage bias applied at the

conducting metal band. The measurement of dose is related back to the threshold voltage

(voltage required to allow current to flow through the semiconductor).(26,27) As the ionizing

radiation travels through the MOSFET, charges generated within are trapped, causing the

11threshold voltage to shift proportionally to deposited dose.(26,27) MOSFETs can be used as real-

time dosimeters(27) and are much smaller than diodes (some are as small as 1 mm in diameter,

known as micro-MOSFETs), decreasing beam attenuation and dose perturbation effects that are

observed for diodes.(26) Their small size also allows for dosimetry in small beams, down to a few

mm (4.4 mm) in beam diameter.(28) Having high Z components and not being water-equivalent,

MOSFETs have large differences in calibration factors,(29) and require separate calibrations to be

performed at different beam energies.(26) On the other hand, they show good agreement with the

ion chamber for a given energy down to a depth of 34 cm.(30) They have also been shown to

exhibit directional anisotropies in response because of the silicon substrate beneath the sensitive

volume,(28,31) and, like diodes, are known to exhibit temperature dependence.(26)

C. Optical Methods

Energy independence of a dosimeter can in large part be met by staying clear of metallic

components within the dosimeter. As such, there has been a considerable amount of effort over

the last few decades to find a dosimeter based on optical characteristics of a radiation sensitive

medium and fibre-optic readout. Among such radiation sensitive media are doped optical

fibres,(32-34) plastic scintillators,(35-39) thermoluminescent dosimeters (TLDs),(34,40) optically

stimulated luminescent (OSL) dosimeters,(34,41,42) and a fluorescing ruby.(43) The media can be

subdivided into two categories: light emitters and light modifiers. Light emitting dosimeters

(such as scintillators, TLDs and OSL media) produce signal that is proportional to the absorbed

dose. Thus the number of photons and the signal statistics are dependent on dose, and is out of

user’s control. On the other hand, light modifying dosimeters alter some aspect of the

interrogating light, the properties and the intensity of which is controlled by the user. This

allows for higher precision measurements, because the number of photons can be increased if the

noise is too high. The other major difference between the two types of optical dosimeters is that

the light emitting media are reusable, whereas light modifying media have to be disposed off

after a certain dose. Reusable dosimeters are often less expensive per use, but “age” and one

must be careful to not assume the signal per dose remains constant as the total dose delivered is

increased. Dosimeters that integrate dose to give a single signal at the end, such as light

modifying media, have the ability to always keep the reading, and can be measured multiple

times as the read-out is non-destructive. As they are also disposable, the need for disinfecting

between patients is eliminated, simplifying their use, as well as reducing the risk of spreading

infection.

12Fluorescing rubies and scintillating fibres automatically emit light when exposed to

ionizing radiation, whereas TLDs and OSL dosimeters must be stimulated by either heat or light,

respectively, to obtain a light signal. These materials work by trapping electrons in higher

energy states when they are exposed to ionizing radiation. When the electrons move down

(either automatically or due to stimulation) to their ground state, photons corresponding to the

energy difference between the two states are emitted. TLDs cannot be read out in real-time, as

they require annealing after irradiation and a lengthy read-out processes for accurate dose

estimate.(5) When one considers the high temperatures that TLDs must be heated to (at least 100

°C, depending on the type of TLD),(28,40) it is hard to imagine how this would be done safely

within a patient. Some fibre-based read-out schemes have been suggested,(40) but have not been

implemented clinically. OSL and plastic scintillator dosimeters are promising, and some have

been made to be nearly energy independent.(38) However, they continue to suffer from

interferences such as fibre scintillation and Cerenkov radiation, where removal of the latter often

requires accurate knowledge of pulse sequences and careful timing.(42,44,45) The alternative is to

use scintillators that have high emission wavelength, such that the Cerenkov radiation (which

drops off with 1/λ3) from the fibre doesn’t interfere much with the dose-related signal from the

sensor.(37) However, the signal per given dose from such scintillators is generally decreased

compare to the signal from scintillators with low emission wavelength, and thus measurements

of dose are noisy.(37) While rubies fluoresce at high enough wavelength and long enough after

the pulse such that the Cerenkov radiation is irrelevant to the measurement, they have a high Z

and are not water equivalent.(43)

Light modifying dosimeters, such as doped optical fibres and Fricke xylenol-orange

solutions or gels create light-absorbing colour centres when exposed to ionizing radiation. This

is done either via electron trapping,(46) or via formation of a complex with a dye,(47) respectively.

Doped optical fibres are generally not water equivalent due to high Z (often Pb) components

used as doping material.(33,34,46) A method for reducing Z by incorporating dopants such as Na,

Mg, and Li has been proposed.(48) However, these optical fibres have not been implemented,

likely because of reduced sensitivity compared to higher-Z counterparts. Finally, while certain

gels can be used as optical dosimeters, these are typically utilized in 3D dosimetry by making 3D

phantoms out of the gel,(49-52) and no effort to incorporate them in fibre-optic dosimeter has been

made; rather, they are used in post-exposure volumetric readout (e.g. MR).

13

V. Outline of Thesis

Another type of optical dosimeter makes use of what is known as a radiochromic medium. This

type of material changes colour, or gets darker, upon exposure to ionizing radiation, and is in the

category of light modifiers. Some radiochromic films are manufactured under the name of

GafChromic® (International Specialty Products, or ISP). These films contain one or two gelatin

layers with organic monomers arranged in a small crystal or micelle-like structure suspended

within them.(53-55) The monomers undergo polymerization when exposed to radiation. The

absorbance spectrum of the resulting polymer systems is then related back to the absorbed

dose.(55-57) Historically, these films are used for two-dimensional dose distribution

measurements,(58,59) and the measurements are performed 3-24 hours (depending on the

film)(54,55) after the end of irradiation to ensure stable readout. This is because the

polymerization reaction is not instantaneous, and proceeds even after the source of ionizing

radiation is removed. This, in turn, causes the absorbance to change with time, producing errors

in dose estimate. (59-61)

Despite the recommendation that these media be read out 3-24 hours after irradiation,

radiochromic media are being investigated in this work for applicability in real-time patient

dosimetry. They have some advantages, including a near water-equivalent organic

composition,(54,55) and the ability to produce signal from a sub cubic millimetre volume.(55) They

also absorb predominantly in the red region of the visible spectrum, where Cerenkov radiation

does not interfere. Because the radiochromic material is a light modifier, the signal statistics can

in part be controlled by the user, by increasing or decreasing the interrogation light. More

importantly, if the performance of these systems during and after irradiation can be characterized

and accounted for, they may provide real-time dose estimates with an acceptable error despite

the above-mentioned issues. If these systems are understood, reverse engineering may be

possible to create a radiochormic material that polymerizes faster and has appropriate sensitivity

for a given application.

In the present work, response of two films (GafChromic MD-55 and EBT) were assessed as

a function of dose, time, dose rate, temperature, and energy and results are described in Chapters

2 to 6. Chapter 2 investigates in detail the possibility of using the radiochromic material,

GafChromic MD-55, as a point-based dose measurement material in real-time. Although

throughout the experiments described in this thesis the measurements were made immediately

after the end of irradiation, the endpoint was chosen only because this is when the dose is known.

It is easy to imagine how, once the relationship between optical density and dose is established,

14optical density measurements can be made any time during irradiation. Thus, the dose can be

estimated during irradiation as well, making it a true real-time dosimeter.

The film was investigated for signal linearity, reproducibility, dose rate and temperature

dependence. Chapter 3 compares the performance of GafChromic MD-55 with a medium from

another film, GafChromic EBT. This chapter focuses on differences in sensitivity and linearity

of MD-55 and EBT, and discusses the fundamental chemical and structural difference between

the two monomer systems. Chapter 4 describes the investigation and quantification of the dose

rate dependence of GafChromic EBT. Chapter 5 describes temperature and humidity

investigations of GafChromic EBT (performed in collaboration with Dr. D.F. Lewis and Dr. S.

Varma, researchers of ISP). Chapter 6 compares energy dependence between two sensitive

media present in three films (MD-55, EBT and HS, where MD-55 and HS use the same

formulation). A novel mathematical model of the response to dose with time both during and for

short periods after the end of irradiation is also developed, with the preliminary results described

in Chapter 7. The parameters of the model are based on physical properties and processes

occurring during exposure to ionizing radiation and interrogation with read-out light. Ideally,

this would allow for future engineering or selection of radiochromic media that meet the in vivo

requirements, by working backwards from the desired response to radiation as predicted by the

model to physical and chemical properties. The thesis concludes with a summary of current

investigations and ideas for future work.

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61. Chu R.D.H., Van Dyk G., Lewis D.F., O'Hara K.P.J., Buckland B.W., Dinelle F.

GafChromic dosimetry media: A new high dose, thin film routine dosimeter and dose

mapping tool. Radiation Physics and Chemistry 35: 767-73, 1990.

21

CHAPTER 2: REAL-TIME RESPONSE OF GAFCHROMIC® MD-55 FILM TO IONIZING RADIATION

Portions of the following have been published as “Suitability of radiochromic medium for real-

time optical measurements of ionizing radiation dose” by Alexandra Rink, I. Alex Vitkin, and

David A. Jaffray in Medical Physics 32(4), p. 1140-1155 (2005)

22

I. Introduction

The goal of these investigations is to develop a dosimeter that is economically and logistically

acceptable (low-cost, disposable, reusable, and sterilizable). To meet these requirements, a water

equivalent dosimeter which undergoes an immediate change in optical properties upon exposure

to ionizing radiation is proposed. The difference in a particular quantitative optical property is to

be measured via optical fibers, and ionizing radiation dose is to be inferred through a calibration

model. In the initial embodiment of the device, the radiation sensitive material present in

GafChromic® MD-55 radiochromic film was investigated for suitability in the application of

external beam patient dosimetry. A list of requirements in choosing an in vivo dosimeter against

which GafChromic MD-55 film is investigated is given in Table 2.

Review of GafChromic® MD-55

To better understand the results presented in this paper, the reader is provided with a

review of literature and explanation of solid-state polymerization. An understanding of the

process that forms the basis for radiochromic dosimetry is required if the real-time dosimetry

system is to be quantified and optimized. The time course of the energy transfer and subsequent

processes that lead to changes in optical density are also important for rational design.

General Experience

Radiochromic films have been used for nearly 30 years in the field of dosimetry.(1)

Commercially available radiochromic dosimeters are manufactured by International Specialty

Products (ISP), and some are sold under the product name of GafChromic® MD-55. A broad

assessment of its characteristics suggest that it is a good candidate for the proposed point-based

dosimeter: the sensitive medium from GafChromic® MD-55 film can be packaged as a small

volume placed at the tip of an optical fiber (closed system to minimize any interference from the

tissue, such as humidity); it has response characteristics within 5% of water and striated muscle

for photons of energy in the range of 0.1-10 MeV, and electrons in range of 0.01-30 MeV.(2)

Upon exposure to heat, ultraviolet (UV) light, and high-energy photons and electrons, the

monomers polymerize to provide an absorbance spectrum with two peaks (675 and 615 nm),

creating a polymer with a blue tint.(2-4) The signal linearity requirement appears to have also

been met, since the change in absorbance is a linear function of the absorbed dose,(5) although the

dynamic range of this function depends on the wavelength at which the measurements are

23Table 2. Evaluation criteria for in vivo point-based real-time dosimeter.

Criterion

#

Criterion Comments

1 small size(6)

(

24obtained.(7-11)

The film has been reported to resolve dose to 1.5 Gy with a precision of 5% or better,

using the 671 nm absorption peak,∗ and this resolution can be further increased by increasing the

thickness of sensitive layer.(12) The requirement of real-time readout appears to be a significant

impediment to use of GafChromic® MD-55 film. Time frames of 24 – 48 hours after exposure

are recommended. Additional investigations are required to resolve this issue.

Less than 5% difference in net change in absorbance of GafChromic® MD-55 film

exposed to 10 Gy at dose rates of 0.034-3.422 Gy/min is expected.∗ However, validity of the

measurements done with this film has been questioned for low dose-rate brachytherapy. Ali and

colleagues reported in 2003 the kinetics of film darkening as function of post-exposure time

depends on the total dose, with the development being faster at the lower doses.(13) These

findings are a concern for real-time dosimetry and require further investigation. While the focus

of this study is to apply the dosimeter in the context of external beam radiotherapy, where dose-

rates are typically greater than those in brachytherapy, a range of doses and dose-rates, over

which post-exposure development from the first few fractions of the treatment does not introduce

error in the absorbance reading and final dose estimate, should be clearly defined.

McLaughlin et al. [1996] reported that propagation of the polymerization is complete

within 2 ms of a single 20 Gy 50 ns pulse.(4) It is unclear, however, if the polymerization

occurred mostly due to ionizing radiation or heat. There literature describes a continuous

increase in absorbance even after irradiation is complete,(14,15) with the absorbance being a

function of a logarithm of elapsed time.(16) Hence, it has generally been recommended to

perform the measurements 24 h (2) to 48 h later(16) by both researchers and manufacturers.*

Measurements are further complicated by the shift in wavelength of maximum absorbance (λmax)

to lower wavelengths as dose increases.(2,7,14,15)

GafChromic® MD-55 film is stable during storage or short exposures to ambient light,(17)

satisfying part of seventh requirement of in vivo dosimeter. However, the temperature

dependence of absorbance of GafChromic® MD-55 is complicated, and humidity and pressure

dependence poorly documented. Increase in temperature reported to correspond to a decrease in

absorbance and a peak shift to lower λ (λmax = 677.5 nm at 18.6°C, 673 nm at 28.0 °C for 6.9

Gy),(2,16) with the latter effect being reversible if temperature fluctuations occur during

∗ International Specialty Products (ISP) product information.

25measurement, not but irreversible if the temperature was varied during irradiation.(2) Others

report an increase in absorbance with an increase in temperature.(14,18) This discrepancy is likely

due to a choice of wavelength for absorbance measurements, as the λmax depends on temperature,

and also due to a range of temperatures sampled. It has been shown that a He-Ne laser operating

as low as 0.1 mW will cause an increase in absorbance of GafChromic® MD-55 in five minutes,

with this effect being stronger for films exposed to smaller doses.(19) For this reason, the

absorbance measurements should be performed using low optical powers to prevent

polymerization due to the heat produced by the light. Above 60°C, the colour of the film

changes from blue to red, as the

point-based ionizing radiation dosimetry using radiochromic materials and … · 2013. 11. 8. ·...

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