Small Field Dosimetric Measurements with TLD-100, Alanine, and Ionization Chambers

Transcription

1 Small Field Dosimetric Measurements with TLD-1, Alanine, and Ionization Chambers S. Junell a, L. DeWerd a, M. Saiful Huq b, J. Novotny Jr. b, M. uader b, M.F. Desrosiers c, G. Bednarz b a Department of Medical Physics, University of Wisconsin, Madison, Wisconsin 5376 b University of Pittsburgh Cancer Institute, Pittsburgh, Pennsylvania c National Institute of Standards and Technology, Gaithersburg, USA IAEA International Symposium on Standards, Applications and uality Assurance in Medical Radiation Dosimetry November 1, 21

2 Introduction Established Broad Beam Codes of Practice (CoPs) American Association of Physicists in Medicine (AAPM) TG-51 and International Atomic Energy Agency (IAEA) TRS-398 IAEA/AAPM working group has recently published a new formalism proposal 1 [1] R. Alfonso, P. Andreo, R. Capote, M. S. Huq, W. Kilby, P. Kjall, T. R. Mackie, H. Palmans, K. Rosser, J. Seuntjens, W. Ullrich, and S. Vatnitsky, A new formalism for reference dosimetry of small and nonstandard fields, Med. Phys. 35, (28). 2/2

3 Purpose Measurement each of variable of beam quality change correction in the calibration factors factors condition for ionization for ionization chambers chambers for multiple possible One calibration possible transfer calibration pathways transfer Proposed small pathway field Standard 6 Co calibration reference conditions Standard CoP calibration conditions calibration conditions 3/2

4 METHODS Nomenclature Dosimeters Phase 1 measurements: In-water measurements of static small fields Phase 2 measurements: In-phantom measurements of static and composite fields

5 Formalism nomenclature: Nomenclature D f w, msr or pcsr msr or pcsr f msr or pcsr = M N msr or pcsr D w k, k f msr or pcsr msr or pcsr, f, ref Combined into a single quality correction factor for this work k f, f = smallfields, f small fields, f k small fields = ( D / M f small fields f w, small fields, f f ( Dw, / M small fields small fields ) ) 5/2

6 Dosimeters Beam quality correction factors were determined experimentally for 3 ionization chambers: Farmer-type Exradin A19.63 cm 3 nominal collecting volume Thimble Exradin A1SL.57 cm 3 nominal collecting volume Microchamber Exradin A16.7 cm 3 nominal collecting volume (Standard Imaging, Inc., Middleton, WI) 6/2

7 Dosimeters Alanine and TLD were used to determine absorbed dose to water Alanine pellets LiF TLD-1 (LiF:Mg,Ti) thermoluminescent dosimeters and aluminum annealing tray. Virtual Water TLD holder 7/2

8 Phase 1 Measurements: Measurements In-water Static small fields (1.6 x 1.6) cm 2 to (1 x 1) cm 2 Dosimeters Ionization chambers Alanine and TLD to determine absorbed dose to water 8/2

9 Phase 2 Measurements: In-phantom measurements of beam quality correction factor of static and composite fields 9/2

10 Phase 2 Measurements: In-phantom measurements of static and composite fields Static field and composite field measurements were performed in a specially-developed cylindrical acrylic phantom PCSR field plan represented a typical IMRT head and neck treatment TLD was used to determine absorbed dose to water (Standard Imaging, Inc., Middleton, WI) 1/2

11 RESULTS Phase 1 results: In-water measurements of beam quality correction factor of static small fields Phase 2 results: In-phantom measurements of beam quality correction factor of static and composite fields

12 Phase 1 results: In-water measurements of beam quality correction factor of static small fields Exradin A19 Farmer-type ionization chamber (.63 cm 3 nominal collecting volume) 12/2

13 Phase 1 results: In-water measurements of beam quality correction factor of static small fields Exradin A1SL thimble ionization chamber (.57 cm 3 nominal collecting volume) 13/2

14 Phase 1 results: In-water measurements of beam quality correction factor of static small fields Exradin A16 Microchamber (.7 cm 3 nominal collecting volume). 14/2

15 Alanine Table 1. Static field alanine pellet irradiation results. Alanine pellet No. Field size Dose measured by NIST Difference from 4 Gy [cm 2 ] [Gy] [%] 1 1.6x x x x x x /2

16 Phase 2 results: In-phantom measurements of beam quality correction factor of static and composite fields 16/2

17 Phase 2 results: In-phantom measurements of beam quality correction factor of static and composite fields Calibration conditions A16 A1SL A19 Setup # Energy Field Size (cm 2 ) SSD/Depth (cm/cm) Medium fsmall fields, f σ f fields σ σ k, small, f f fields k, small, f k, 1 6 Co 1 x 1 1/5 water a 6 Co 4.9 x 4.9 1/5 water b 6 Co 1 x 1 1/1 water b 6 Co 1 x 1 1/na phantom c 4a 6 Co 4.9 x 4.9 1/na phantom a 6 Co 4.9 x 4.9 1/1 water c 6 MV 1 x 1 1/1 water c 6 MV 1 x 1 1/na phantom b 6 MV 2 x 2 1/1 water MV 2 x 2 1/na phantom MV f pcsr 1/na phantom Table 2. Measured beam quality correction factor 17/2

18 Phase 2 results: In-phantom measurements of beam quality correction factor of static and composite fields Calibration conditions A16 A1SL A19 Setup # Energy Field Size (cm 2 ) SSD/Depth (cm/cm) Medium fsmall fields, f σ f fields σ σ k, small, f f fields k, small, f k, 1 6 Co 1 x 1 1/5 water a 6 Co 4.9 x 4.9 1/5 water b 6 Co 1 x 1 1/1 water b 6 Co 1 x 1 1/na phantom a 6 Co 4.9 x 4.9 1/na phantom a 6 Co 4.9 x 4.9 1/1 water c 6 MV 1 x 1 1/1 water c 6 MV 1 x 1 1/na phantom b 6 MV 2 x 2 1/1 water MV 2 x 2 1/na phantom MV f pcsr 1/na phantom Table 2. Measured beam quality correction factor 18/2

19 TLD-1 Average standard deviation of ~1.5% Estimated relative combined standard uncertainty ~3 4% (k=1) Alanine Uncertainty Average standard deviation of ~5% For composite fields and single static fields the precisions of TLDs and alanine remained constant 19/2

20 Conclusions The beam quality correction factors were unity within statistical uncertainties of the dosimeters for: All calibration conditions for the thimble and small volume 9 of 11 calibration conditions for the Farmer-type Future beam quality correction factor studies Determine best calibration condition for determining calibration coefficient Optimal application of calibration coefficient to proposed protocol Monte Carlo simulations using measurements as benchmarks 2/2

21 Thank You National Institute of Standards and Technology University of Pittsburgh Cancer Institute Brian Hooten and Standard Imaging Inc. UW MRRC staff and students UW Radiation Calibration Lab customers for their ongoing support of student research