SU-E-T-15: Small and Nonstandard Photon Field Dosimetry Characterization Using Monte Carlo Methods

Medical Physics
S Junell, L DeWerd

Abstract

To quantify uncertainty reduction in small photon field dosimetry through characterization of ionization chambers and calibration conditions using detailed Monte Carlo methods benchmarked against NIST-traceable measurements. Phase space profiles were obtained using detailed EGSnrc Monte Carlo models for a Varian 6 MV photon linear accelerator, and a NIST-traceable cobalt-60 teletherapy unit. The responses of a farmer-type ionization chamber, two micro-ionization chambers, and one scanning-type ionization chamber were simulated in multiple calibration conditions. Calibration conditions included static field sizes ranging from (10×10) cm squared to (0.5×0.5) cm squared for the 6 MV and cobalt-60 beam qualities and an additional dynamic IMRT plan for the 6 MV beam quality. Calibration conditions also consisted of ionization chambers placed in a standard water phantom and in a specially designed acrylic phantom. Tolerance limits on the calibration conditions were investigated. All models were benchmarked against measured beam quality data, including ionization chamber beam quality correction factors for the standard absorbed dose to water cobalt-60 calibration coefficient. The majority of the simulated small field response values f...Continue Reading

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