Advanced Dosimetry Techniques

for Accurate Verification of

Nonstandard Beams

Eunah Chung1, Emilie Soisson1,2, Hugo Bouchard3,4, and Jan Seuntjens1

1Medical Physics Unit, McGill University, Montréal, Québec, Canada

2Dept. of Medical Physics, McGill University Health Centre, Montréal, Québec, Canada

3Département de Physique, Université de Montréal, Montréal, Québec, Canada

4Département de Radio-Oncologie, Centre Hospitalier de l’Université de Montréal,

Montréal, Québec, Canada

2010 IAEA IDOS Symposium, Vienna, Austria

November 10th, 2010

Introduction

� Common use of small and composite non-uniform fields

in radiotherapy

� IMRT, tomotherapy, cyberknife, gammaknife, etc.

� Standard dosimetry protocols

� AAPM TG-51(1) and IAEA TRS-398(2)

� Large dosimetric errors for small and non-uniform field

calibration

(1) Almond et al., Med. Phys. 26:1847 (1999)(2) Andreo et al., International Atomic Energy Agency, Vienna, Austria (2000)

Fraser et al., J. Appl. Clin. Med. Phys. 10: 243 (2009)

Introduction

� New methods required for accurate beam calibration in small and non-uniform (nonstandard) fields

� Common use of small and composite non-uniform fields

in radiotherapy

� IMRT, tomotherapy, cyberknife, gammaknife, etc.

� Standard dosimetry protocols

� AAPM TG-51(1) and IAEA TRS-398(2)

� Large dosimetric errors for small and non-uniform field

calibration

(1) Almond et al., Med. Phys. 26:1847 (1999)(2) Andreo et al., International Atomic Energy Agency, Vienna, Austria (2000)

Objectives

� Apply established dosimetry techniques(3) for accurate

dose measurements in various small and nonstandard

fields.

� Determine the correction factors for these fields based on

the new dosimetry formalism.

(3) Chung et al., Med. Phys., 37:2404 (2010)

Reference fieldf ref

Machine specificreference field fmsr

Plan-class specific reference field fpcsr

(4) Stewart et al., Phys. Med. Biol., 52: 3089 (2007)

(4)Gafchromic ®

EBT filmDiamonddetector

GLIC-03 microLion(PTW)

Chung et al. Med. Phys. 37:2404 (2010)

(c)TomoTherapy®

TomoTherapy®

(c)TomoTherapy®

85 cm

85 cm

Thomas et al., Med. Phys. 32, 1346-1353 (2005)

Characterization of reference detectors

Method: Bouchard et al., Med. Phys., 36:1931 (2008) Chung et al., Med. Phys. 37:3265 (2010)

Chung et al. Proc.

of IAEA IDOS,

submitted (2010)

Correction factor

(5)

(5) Med. Phys. 32:570-577 (2005)

Farmer-type chambers smaller ionization chambers

( fmsr= 5×10 cm2 )

Candidate pcsr fields

� A TomoTherapy®-based IMRT delivery

VolumeSize

(cm3)

Prescribed

Dose (Gy)Importance

DVH

Vol. (%)

Ave. (1SD)

(Gy)

Homogeneity

Index (HI)

PTV-HD 78.42 70 300 97 71.09 (0.45) 0.025

PTV-ED 585.22 56 300 97 58.78 (3.40) 0.248

spinal

cord21.00 25 50 50 26.55 (12.19) 1.432

40 % (28 Gy)50 % (35 Gy)60 % (42 Gy)70 % (49 Gy)80 % (56 Gy)90 % (63 Gy)100 % (70 Gy)

A

P

R L R L

S

I I

S

A P

PTV-HDPTV-ED

spinal cordaxial coronal sagittal

Ionization

ChamberExradin A12 NE2571

Exradin

A1SLExradin A14

PinPoint®

31006

Measurement

1

0.995

±0.0040.997

±0.0041.001

±0.0040.994

±0.0040.997

±0.004

Measurement

2

0.995

±0.0030.993

±0.0030.993

±0.0031.002

±0.0030.994

±0.004

Correction factor

Meas. 1

Meas. 2

Farmer-type chambers smaller ionization chambers

Chung et al. Proc.

of IAEA IDOS,

submitted (2010)

Various nonstandard field deliveries

100 %: Dave at the small PTV

20 %30 %40 %50 %60 %70 %80 %90 %95 %100 %

small PTV

Correction factor

Correction factor

Correction factor

Conclusions

� Correction factor measurement: within 0.4% 1σ uncertainty

� Msr correction factor

� agreed well between the measurement and calculation for

Farmer-type chambers

� Pcsr correction factor

� close to unity for homogeneous dose delivery

� large positioning uncertainty for heterogeneous dose delivery

� This work suggests the use of a qualifier of plan

homogeneity such as HI or MF to uniquely specify pcsr

correction factors.

Acknowledgements

� Pierre Léger

� Robin van Gils

� Joe Larkin

� McGill MPU staff and students

E.C.: John McCrae Fellowship(Faculty of Medicine)

Grant No. RGPIN 298181H.B.: Doctoral Scholarship

E.C.: Research Institute studentship

E.C.: ICR travel award