Small Field Dosimetry: Overview of AAPM TG-155 and the IAEA-AAPM Code of Practice (Therapy) IJDas (1) Indra J. Das, PhD, FAAPM, FACR, FASTRO Department of Radiation Oncology Indiana University School of Medicine Indianapolis, Indiana, USA Wildflower, Panama
Cone Factor (St) Small Field Dosimetry Problem 1.02 Total scatter factor from different institutions 1.00 0.98 0.96 0.94 0.92 0.90 0.88 0.86 0.84 12% diff WEH U-Arizona Temple U U Penn Erlanger Serago et al. Fan et al. Zhu et al. Institutional variability in 6 MV Radionics SRS dosimetry 0.82 0.80 10 15 20 25 Cone Diameter (mm) 30 35 40 IJDas (2) Das et al, J Radiosurgery, 3, 177-186, 2000
Cone Factor (St) Dosimetric Variation with Detectors 1.00 Total scatter factor with various detectors 0.98 0.96 0.94 0.92 0.90 0.88 0.86 0.84 0.82 14% Diamond CEA (Film) Kodak(Film) TLD Pinpoint(par) Pinpoint(per) 0.125ion(par) 0.125ion(per) MC(1mm) MC(5mm) IJDas (3) 0.80 10 15 20 25 30 Cone Diameter (mm) Das et al, J Radiosurgery, 3, 177-186, 2000 35 40
What is a Small Field? Lack of charged particle Dependent on the range of secondary electrons Photon energy Collimator setting that obstructs the source size Detector is comparable to the field size IJDas (4)
Definition of Small Fields IJDas (5) Das et al, Med. Phys. 35, 206-215, 2008
Energy FluenceSc normalized Energy Fluence Penumbra/Output 1.0 0.8 1.0 0.8 Source sizes cm ---- 0.01 ---- 0.35 ---- 0.50 0.6 0.6 0.4 0.2 Large field 0.4 0.2 2.0x2.0 1.0x1.0 cm 0.5x0.5 cm 2 22 0.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 x cm 0.0 1.5 1.0 0.5 0.0 0.5 1.0 1.5 x cm IJDas (6) Nyholm et al Radiother Oncol 78, 347, 2006 and Phys Med Biol 51,6245, 2006
Source Size IJDas (7) 90%, 70%, 50%, 30%, 10% iso-intensity line Jaffray et al, Med Phys 20, 1417-1427 (1993)
Source Size, Modern Machines Varian TrueBeam T1 T2 T3 6 MV Dimensions x, y (mm) 0.9, 0.7 6 MV FFF 0.8, 0.7 10 MV 0.9, 0.9 10 MV FFF 0.9, 0.8 IJDas (8) Sawkey et al, Med Phys, 40, 332, 2013
IAEA/AAPM Proposed Pathway IJDas (9) Alfonso, et al. Med Phys 35, 5179-5186 (2008)
Why So Much of Fuss? Reference (ref) conditions cannot be achieved for most SRS devices (cyberknife, gammaknife, tomotherapy etc) Machine Specific reference (msr) needs to be linked to ref Ratio of reading (PDD, TMR, Output etc) is not the same as ratio of dose D1 D 2 M M D 1 M 1 f clin, f k Q clin, Q D2 M 2 1 2 msr msr IJDas (10)
Relative Dosimetry D f w, Q m s r m s r M f Q m s r m s r N DW, Qo k Q, Qo k f Q m s r m s r,, f Q ref f Q clin clin f Q msr msr M M f Q f Q clin clin msr msr f clin fclin D w, Q M clin Qclin f msr f msr D / M w, Q msr fclin / M Qclin fclin, f msr Q msr M f Q msr msr k Q clin, Q msr k f Q clin f msr f clin Dw, Q / M Q ( Output) clin w, clin rel f f D / M clin clin (Re ading rel, f msr, Q msr w, Q w, Q clin ) msr msr IJDas (11) k f Q clin clin, f, Q msr msr ( S ( S w, air w, air ) ) fclin fmsr P P fclin msr
IJDas (12) Meaning of k in Micro-Chambers
IJDas (13) Stopping Power Ratio
Radiological Parameters IJDas (14) Sanchez-Deblado et al, Phy. Med. Biol., 48, 2081, 2003
Cyber Knife Dosimetry 0.5% IJDas (15) F. Araki, Med. Phys. 33, 2955-2963 (2006).
Correction Factors Correction Factor depends on: Field size Source size (FWHM) Detector type IJDas (16) Francescon, et al Med Phys 35, 504, 2008
Correction factor IJDas (17) 1.16 1.14 1.12 1.10 1.08 1.06 1.04 1.02 1.00 0.98 0.96 0.94 0.92 0.90 Published data on 0 5 10 15 20 25 30 35 Field size (mm) Francescon et al Med Phys,38, 6513, 2011 Siemens; PTW diode 60012 Elekta; PTW diode 60012 Siemens; Exradin A16 Elekta; Exradin A16 Siemens; Sun Nuclear Dedge "Elekta; Sun Nuclear Dedge" Siemens; PTW Pinpoint 31014 Elekta; PTW Pinpoint 31014 Siemens; PTW microlion Elekta; PTW microlion
IJDas (18) Med. Phys. 38 (12), 6992, 2011 of Linear Accelerators (Varian)
of Linear Accelerators Med. Phys. 38 (12), 6513-6527, 2011 IJDas (19) Radiotherapy and Oncology 100 (2011) 429 435
Cyber Knife IJDas (20) Pantelis et al, Med Phy. 37, 2369-2379, 2010
Uncorrected output difference (%) with MC CyberKnife data 12 10 8 6 4 2 0-2 -4-6 -8-10 -12 5 mm cone PinPoint Diamond(old) EBT2 MicroTLD IBA-SFD PTW Diode SN-Edge IJDas (21) Moignier et al, Med Phys 41(7), 071702, 2014
IJDas (22)
IJDas (23) Med Phys, 41(4), 041711, 2014
Newer Data on Benmakhlouf et al, Med Phys, 41(4), 041711, 2014 IJDas (24)
IJDas (25) New Published data From IAEA
IJDas (26) Correction Factors
IJDas (27)
Correction in Profile (OAR) 5 mm cone 25 mm cone IJDas (28) Francescon et al, Med Phys, 41(10), 101708, 2014
Depth Dose & Source Size IJDas (29) Sham et al, Med Phys, 35, 3317-3330, 2008
Good News with Modern Microdetectors Papaconstadopoulos, et al, Phys. Med. Biol. 59 (2014) 5937 5952, IJDas (30)
IJDas (31)
Machine, Cone & Depth? Radiotherapy Oncology, 112, 442-446, 2014 IJDas (32)
Validity of Proposed Method IJDas (33) Relative dose at d max 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 1 2 3 4 5 6 Field Size (cm)
Summary Understand the limit of small field Focal spot issues Detectors that are water equivalent like, MicroLion, MicroDiamond and Plastic Scintillators are best suited Use proper correction factors to correct detector response to correct for the dose IJDas (34) Watch for IAEA and TG-155 guidelines when they are published
Sam Question # 1 What is a small field in photon beam dosimetry? 6% A. Less than 3x3 cm2 25% 2% 4% 63% B. Less than 2x2 cm 2 C. Depends on type of machine (CyberKnife, Gamma Knife, etc) D. Depends on detector (pinpoint, microdiamond, micro-diode etc) E. Depends on beam energy and source size AAPM-2015/ IJDas (35)
Sam- Answer Answer: E - Depends on Beam Energy and Source Size Ref: Das et al, Small fields: Non-equilibrium radiation dosimetry," Med Phys 35, 206-215 (2008) AAPM-2015/ IJDas (36)
Sam Question # 2 The source size of a linear accelerator is typically? 0% A. 20 mm 4% 3% 15% 78% B. 10 mm C. 5 mm D. 2 mm E. 1-10 mm, depending on the machine AAPM-2015/ IJDas (37)
Sam- Answer 2 Answer: E Depending on machine Ref: Jaffray et al, Med Phys 20, 1417-1427 (1993) AAPM-2015/ IJDas (38)
Sam Question # 3 Dosimetry of small field should be performed with; A. Small water-equivalent detectors (MicroLion, MicroDiamond, Plastic Scintillators) B. Edge detector C. Pinpoint D. MicroMosfet E. Any micro-detector 89% 2% 1% 0% 9% A. B. C. D. E. AAPM-2015/ IJDas (39)
Sam- Answer #3 Answer: A The detectors that are water equivalent Ref: Azangwe et al, Med Phys 41, 072103 (2014) Benmakhlouf et al, Med Phys 41, 041711 (2014) AAPM-2015/ IJDas (40)
IJDas (41) Thanks