Development of An Independent, Monte Carlo, Dose Calculation, Quality Assurance Tool for Clinical Trials
A Faught1*, S Davidson2, J Fontenot3, S Kry4, C Etzel5, G Ibbott6, D Followill7, (1) M.D. Anderson Cancer Center, Houston, TX, (2) MD Anderson Regional Care Centers, Nassau Bay, TX, (3) Mary Bird Perkins Cancer Center, Baton Rouge, LA, (4) MD Anderson Cancer Ctr., Houston, TX, (5) The University of Texas Graduate School of Biomedical Sciences, Houston, Texas, (6) UT MD Anderson Cancer Center, Houston, TX, (7) UT MD Anderson Cancer Center, Houston, TXSU-E-T-159 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To commission a multiple-source Monte Carlo model of Elekta linear accelerator beams of nominal energies 6MV and 10MV.
Methods: A three source, Monte Carlo model of Elekta 6 and 10MV therapeutic x-ray beams was developed in a two-step process. Energy spectra of each of three sources, a primary source corresponding to photons created in the target, an extra-focal source corresponding to photons originating from scattered events in the linac head, and an electron contamination source, were determined. The two photon sources were determined by an optimization process that fit the relative fluence of 0.25 MeV energy bins to the product of Fatigue-Life and Fermi functions to match calculated percent depth dose (PDD) data with that measured in water for a 10x10cm² field. Off-axis effects were modeled by fitting the off-axis fluence to a piece-wise linear function through optimization of relative fluence to match calculated dose profiles with measured dose profiles for a 40x40cm² field. A 3rd degree polynomial was used to describe the off-axis half-value layer as a function of off-axis angle. The model was then commissioned by comparing calculated PDDs and dose profiles for field sizes ranging from 3x3cm² to 30x30cm² to those obtained from measurements.
Results: Agreement between calculated and measured data was evaluated using 2%/2mm global gamma criterion for field sizes of 3x3, 5x5, 10x10, 15x15, 20x20, and 30x30cm². Along the central axis of the beam 99.5% and 99.6% of all data passed the criterion for 6 and 10MV models, respectively. Dose profiles at depths of dmax, 5, 10, 20, and 25cm agreed with measured data for 95.4% and 99.2% of data tested for 6 and 10MV models, respectively.
Conclusion: A Monte Carlo multiple-source model for Elekta 6 and 10MV therapeutic x-ray beams has been developed as a quality assurance tool for clinical trials.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by Public Health Service grants CA010953, CA081647, and CA21661 awarded by the National Cancer Institute, United States Department of Health and Human Services.