ROC7080: RADIATION THERAPY PHYSICS LABORATORY

LAB H: TG-51

GROUP I SPRING/SUMMER 2014

KEVIN JORDAN

GRADUATE STUDENT, RADIOLOGICAL PHYSICS KARMANOS CANCER CENTER

WAYNE STATE UNIVERSITY

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Table of Contents

I.   Introduction  &  General  Theory  ...................................................................................  3  II.   Laboratory  Procedures,  Methods  &  Specific  Theory  ..........................................  5  

II.a Photons (6X & 10X)  ....................................................................................................................  5  II.b Electrons (10MeV Cylindrical)  ................................................................................................  6  II.c Electrons Plane Parallel Cross Calibration  ...........................................................................  6  II.d Electrons (6MeV Plane Parallel)  ..............................................................................................  7  

III.   Questions  &  Discussion  ...............................................................................................  8  

IV.          Appendix  C:  References  ............................................................................................  17  

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I. Introduction & General Theory

In this laboratory, the uses and mechanisms of TG-51 have been explored. The proceeding sections can be found to include Laboratory Procedures & Methods, Questions & Discussion along with the appendices, which include the experimental data, sample calculations and references.

Many items are linear between the different portions of the laboratory; e.g. the nature of determining PTP, Pion, Ppol, Pelec are one in the same whether it be photons or electrons, it is just a matter of recording the data for that given setup and the appropriate charge readings for the Pion and Ppol calculations. Some of the general equations for these correction factors for the Mraw charge readings can be found below; Pelec is provided by the ADCL.

Recall, the corrected charge readings can be found by the multiplicative result of all the correction factors.

The proceeding setup figure can be found to represent SSD (left) and SAD

(right) setup; typically 100 cm SSD setup at 10cm deep will be chosen as was the case in this laboratory.

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II. Laboratory Procedures, Methods & Specific Theory II.a Photons (6X & 10X) A nice way to start is looking at the flow chart, first introduced to in ROC7040, pp. 101. It is a great logical framework for working through the photon setups, dependent upon the energy at hand.

The proceeding figure can be found to represent PDI cylindrical unshifted, shifted and PDD. Recall that with photons dose is fairly proportional to charge collected as photon energy does vary greatly with depth, as compared to electrons which do.

 Figure  1.  General  Representation  of  Photon  PDI  &  PDD

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II.b Electrons (10MeV Cylindrical)  

One can easily notice in the right hand graph the disparity between uncorrected PDI (long dashed “I”) vs. PDI chamber shift correct (solid line) vs. the PDD (short-dashed line); notice how graph a has really no shift for photons while electrons do require a shift since dose is not necessarily proportional to charge collected.

II.c Electrons Plane Parallel Cross Calibration

This is an extremely useful method to guarantee linearity and consistency in measurements. Recall that all ionization chamber are calibrated against Cobalt 60, but plane parallel chamber are especially sensitive in their construction that their resulting measurement are not as consistent (repeatable) as their cylindrical counterpart. Therefore, it is suggested in TG-51 to utilize a cross calibration based on results from a high energy electron beam and its respective measurements with a cylindrical chamber.

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II.d Electrons (6MeV Plane Parallel)

Having described II.c for the cross calibration for plane parallel ionization chamber, one can then utilize these results for electron energies below 10 MeV. A physical representation from ROC7040 coursepack is shown below along with derivation for the calibration factor described previously.

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III. Questions & Discussion 1. Describe the reason for AAPM recommendation to switch from TG-21 to

TG-51 calibration protocol. How are the output factors affected for photon and electron beams?

From pp. 121 of the ROC7040 course pack, which serves as a nice summary for the switch to TG-21 to TG-51, one finds that TG-21was complicated, that numerous improvements in the field of dosimetry had been made in the past 15 years, and there were also small errors/inconsistencies in the report. TG-21 was complicated because the physicist would calibrate the ion chamber in air related to exposure and then calibrate the beam in water in terms of absorbed dose. TG-51 asks the physicist to calibrate the ion chamber and the beam in water in terms of absorbed dose, a more intuitive endeavor.

2. How do you specify beam quality for photon and electron beams?

The beam quality specifier for photon is %dd(10)x and electrons use R50 . In graph (a) from TG-51 figures reference, one can see that %dd(10)x is the % ionization at 10 cm deep, in this example it is 77% of dmax. Graph (b) represents the I50 which can then be converted to R50 by referencing the empirical formula from TG-51 below.

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3. What chambers are recommended for photon and electron output measurements and why? What are the typical dimensions for such chambers?

Cylindrical (Farmer) chambers are recommended for all photon beam measurements and for all electron beams above 10 MeV. Cylindrical chambers are the most accurate and repeatable in their design and thus linear results in one’s measurements are most ensured with the cylindrical chamber.

A plane parallel (Markus) chamber should be used for less than 10 MeV electrons because the fluence perturbation will become too large with a cylindrical chamber. The two figures below can be found to show rough dimensions for the two styles of chambers. The active diameter of a typical cylindrical chamber is around 6mm (3mm radius), which would be the values used for consideration when doing rcav shifts for effective point of measurement.

 

Figure  2.  Cylindrical  Ion  Chamber  

 

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Figure  3.  Plane  Parallel  Ion  Chamber

4. What is Kq and where does it come from? Briefly explain the terms included in Kq for photons.

A screenshot derivation can be seen below for kQ.

 

Figure  4.  kQ  derivation  for  photons

Recall that kQ is based on the ionization chamber and beam quality. It is used as converter from the provided calibration constant from the ADCL which is based on Cobalt 60. Obviously the ADCL can not do every calibration based on your unique machine because all machines are unique and will have a different value. Therefore, the physicist will used kQ from the table below based on a water tank scan finding %dd(10)x and the respective chamber used to find the kQ value which will then convert N-Co60 to N(Q;D,w) based on one’s own machine.

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5. Why is Kq for electrons expressed as a product of three terms? Please explain the three terms.

The breakdown of the terms can be seen in steps below.

𝒌𝑸 = 𝑷𝒈𝒓𝑸 𝒌′𝑹𝟓𝟎𝒌𝒆𝒄𝒂𝒍

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The first term is the gradient correction factor for cylindrical chambers, and it depends upon cavity radius and ionization gradient at the point of measurement, which must be determined by the used.

KR50 is a chamber specific factor, which depends on the user’s beam quality. It is made up of kecal and k’R50. K’R50 can be found on the table below based on energy and chamber type.

Kecal can be found on the table below based on the chamber used.

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These factors are nicely summarized in the screenshot below by using some substitution and rearrangement of terms, which shows the dose to water based on one’s beam quality for photon calibration factor become useful as an electron calibration factor.

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6. Why do we do cross-calibration? Describe how you would do it.

A nice pictorial representation can be shown below showing a cylindrical chamber on the left and plane parallel chamber on the right. First, one can find I50, R50 and then dref using, for example, 18 MeV electrons with the cylindrical chamber. In this laboratory dref was found to be 4.577 cm and finish taking the necessary measurements for that.

Then one may proceed to one of the two methods described on pp. 1895 of TG-51, in this laboratory Method A was used. Charge will then be collected at dref using the plane parallel chamber, corrected for as usual. K’R50 can than be found from figures or from the empirical expression. The calibration constant can then be found based on the dose/MU at dref from the cylindrical chamber multiplied by the MU and then call divided by the corrected charge reading and k’R50; as seen in the equation below.

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7. Why do we use dref as the absolute calibration measurement point for electrons?

Recall, the relationship below.

From TG-51, pp. 1851, one may note that by going to this depth the physicist may make use of stopping power ratios appropriately and realistically, instead of assuming that the beam in mono-energetic.

8. Is there a difference between PDI and PDD curve for photons? What about for electrons? If there is a difference, where does it come from?

A general relationship is immediately spelled out in the ROC7040 coursepack chapter on electrons; notice below:

A pictorial representation of these relationships was presented earlier in the theory section, but again show for photons in graph (a) on left and for electrons in graph (b) on the right.

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Another representation of PDI and PDD for 18 MeV electrons can be seen below.

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IV. Appendix C: References TG: 21, 51

Burmeister, Jay. Radiation Dosimetry Coursepack. 2014 Rakowski, Joe. Radiation Therapy Physics Course Notes. 2014