Plan-Specific Correction Factors for Small- Volume Ion Chamber Dosimetry in Modulated Treatments on a Varian Trilogy

Transcription

1 Plan-Specific Correction Factors for Small- Volume Ion Chamber Dosimetry in Modulated Treatments on a Varian Trilogy Vimal K. Desai, M.S. Under the supervision of Dr. Wesley Culberson NCCAAPM 2017 Fall Meeting La Crosse, WI November 10 th, 2017 Medical Radiation Research Center

2 Standard Dosimetry Dosimetric codes of practice (COP) rely on accurate ionization chamber readings Reference conditions for 6 MV Photon Beam 100 cm SSD 10 cm 10 cm field size Depth of 10 cm in water Varian Trilogy linear accelerator 1 1. (Image source) Varian Medical Systems, Palo Alto, CA, USA, 2. (Image source) Standard Imaging,. Middleton, WI, USA, Exradin A12 Farmer-type chamber 2 2

3 Nonstandard Dosimetry Modulated Patient Plans can be far removed from reference conditions Small/nonstandard fields Modulated Machine Parameters Dose Rate Gantry Angle Multileaf Collimator (MLC) fields 3

4 Composite Field Dosimetry Detector response studied in small static fields 1 Less known regarding absolute dosimetry in composite fields Exradin A1SL scanning chamber investigated in numerous clinical plans Exradin A1SL scanning chamber 2 MLC apertures from a clinical plan 1. I. Das, J. Morales, P. Francescon, AIP Conf. Proc (2016) 2. (Image source) Standard Imaging,. Middleton, WI, USA, 4

5 The Path to Clinical Dose Determination Progression from reference to clinical dose determination 1 Reference Conditions Clinical Delivery,, 1. R. Alfonso, P. Andreo, R. Capote et al., Med. Phys. 35 (2008) 5

6 Clinical Plans and QA Geometry Anonymized patient plans from UW Health satellite clinic imported to UW Medical Radiation Research Center (UW-MRRC) Investigated 131 actual IMRT patient plans from a Varian Clinac 21EX linac 92 step-and-shoot plans (717 beams) 39 VMAT plans (117 arcs) Multiple sites: Prostate, Lung, Pelvis, H&N, Pancreas, Brain, Chest wall, Rectum, etc. Exradin A1SL Scanning Chamber 1 QA Geometry: cylindrically symmetric Tomotherapy QA phantom Exradin A1SL at center of phantom 1. (Image source) Standard Imaging,. Middleton, WI, USA, Cylindrical Tomotherapy QA Phantom 6

7 Monte Carlo User Codes Electron Gamma Shower (EGSnrc) user codes 1 : BEAMnrc: Model 21EX linac 2 DOSXYZnrc: Validate modulated dose distributions against Eclipse TM Acuros XB algorithm egs_chamber: Determine A1SL beam-specific corrections A1SL egs_chamber Model 1. I. Kawrakow & D. Rogers, Rad. treat. phys. 27(3) (2000) 2. (Source) T. McCaw, PhD Thesis, University of Wisconsin (2015) Active Volume 7

8 Determination of,, 8

9 Step-and-shoot Individual Beams 91/717 step-and-shoot beams (12.7%) required correction with volume averaging 2% correction benchmark 13/717 step-and-shoot beams (1.8%) required correction without volume averaging Average simulation uncertainty 0.5% with volume averaging, 0.2% without Individual step-and-shoot beams correction magnitudes 9

10 Step-and-shoot Composite Plans 1/92 composite plans (1.1%) required correction with volume averaging 2% correction benchmark 0/92 composite plans (0%) required correction without volume averaging Average simulation uncertainty 0.5% with volume averaging, 0.2% without Step-and-shoot composite plan correction magnitudes 10

11 VMAT Individual Beams 41/117 arcs (35.0%) required correction with volume averaging 2% correction benchmark 24/117 arcs (20.5%) required correction without volume averaging Average propagated simulation uncertainty was 0.8% with volume averaging, 0.4% without VMAT individual beam correction magnitudes 11

12 VMAT Composite Plans 8/39 plans (20.5%) required correction with volume averaging 2% correction benchmark 3/39 plans (7.7%) require correction without volume averaging Average propagated simulation uncertainty was 0.8% with volume averaging, 0.4% without VMAT composite plan correction magnitudes 12

13 Comparison of Modalities VMAT fields requires larger magnitude corrections more frequently than stepand-shoot fields Plan averaging diminishes correction magnitudes 2% correction benchmark VMAT average uncertainty: 0.8% Step-and-shoot average uncertainty: 0.5% All modality correction magnitudes comparison 13

14 Comparison of Modalities (No Volume Effects) Similar trends observed Uniform decrease in total correction without volume effects VMAT average uncertainty: 0.4% 2% correction benchmark Step-and-shoot average uncertainty: 0.2% All modality correction magnitudes comparison excluding volume averaging 14

15 Conclusions and Future Work Composite fields are not static small fields and should not be treated the same VMAT beams cause larger corrections more often than step-andshoot beams Placing A1SL in a region of large composite dose does not guarantee correctionless dosimetry based on reference field calibrations Extend study to more detectors, modalities, and treatment machines 15

16 Acknowledgements Dr. Wesley Culberson Dr. Larry DeWerd UW Carbone Cancer Center Dr. Zacariah Labby UWMRRC students and staff UW Accredited Dosimetry Calibration Lab customers Special thanks to: Dan Anderson Continued support helps fund student research Keith Kunugi John Micka Thank you for your attention! Questions are welcome

17 Detector Response in Small Fields Detectors have been extensively studied in small, static fields 1 Newer detectors available Composite field response is less understood PTW microdiamond 1 A1SL scanning chamber corrections investigated 1. I. Das, J. Morales, P. Francescon, AIP Conf. Proc (2016) 2. (Image source) PTW Freiburg, Freiburg, Germany, 3. (Image source) MediTron, Frauenfeld, Switzerland, Exradin W1 Scintillator 2 17

18 Chamber Perturbations Various chamber perturbations in modulated fields Chamber Wall Central Electrode Stem Air Cavity Central Electrode Wall Air Cavity composition and density (Replacement) Volume Correlated Simulations Chamber Stem Tally cavity dose with and without component present,,

19 Chamber Perturbations Various chamber perturbations in modulated fields Stem Chamber Wall Air Cavity Central Electrode Central Electrode Correlated Simulations Tally cavity dose with and without component present,,,

20 Determination of,,,, 1 Four Simulations for each Beam Clinical Field phantom/a1sl dose Reference Field phantom/a1sl dose Clinical prostate field TPS (left) and egs_chamber (right) Reference Fields (10 x 10) cm 2 jaw-defined field 100 cm SAD setup 1. B.R. Muir and D.W.O. Rogers, Med. Phys. 37, (2010) Reference field TPS (left) and egs_chamber (right) 20