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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
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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)
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Fraser et al., J. Appl. Clin. Med. Phys. 10: 243 (2009)
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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)
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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)
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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)
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Chung et al. Med. Phys. 37:2404 (2010)
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(c)TomoTherapy®
TomoTherapy®
(c)TomoTherapy®
85 cm
85 cm
Thomas et al., Med. Phys. 32, 1346-1353 (2005)
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Characterization of reference detectors
Method: Bouchard et al., Med. Phys., 36:1931 (2008) Chung et al., Med. Phys. 37:3265 (2010)
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Chung et al. Proc.
of IAEA IDOS,
submitted (2010)
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Correction factor
(5)
(5) Med. Phys. 32:570-577 (2005)
Farmer-type chambers smaller ionization chambers
( fmsr= 5×10 cm2 )
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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
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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)
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Various nonstandard field deliveries
100 %: Dave at the small PTV
20 %30 %40 %50 %60 %70 %80 %90 %95 %100 %
small PTV
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Correction factor
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Correction factor
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Correction factor
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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.
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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