SU-E-T-55: A Study of Film Dosimetry for Routine Beam Profile and PDD Constancy Checks in Proton Beams

Abstract

Purpose: Radiographic film dosimetry suffers from energy dependence in Proton dosimetry, and thus is not suitable for absolute dosimetry. In this study, we investigate film dosimetry for the constancy check of percentage depth dose (PDD) and beam profile measurements in proton beams. Methods: From PDD measured by film and ion chamber (IC), calibration factors as a function of depth (IC/film) was obtained. These factors imply variable slopes (with energy and depth) of linear characteristic curves that relate film response to dose. They were used to convert day‐to‐day film measurements into dose. Film dosimetry of a 186 MeV proton beam was performed to investigate this hypothesis. In addition, Monte Carlos simulation of a 250 MeV proton beam was performed calculating proton fluence spectrum along the off‐axis direction. By multiplying stopping powers of film emulsion and water, respectively, to the spectrum, doses to film and water were calculated. The ratio of film dose to water dose was evaluated across the off‐axis distance to understand film response. Results: The measured and calibrated PDD approached to that of IC, but near the end of spread‐out‐bragg‐peak (SBOP), a spurious peak is observed due to the mismatch of distal edge between calibration film and measurement film. The SBOP width was measurable within 1mm. The distal edge was reproducible within 1.5mm. Entrance dose was reproducible within 5.5%. The possible sources of such errors include developer uncertainty, film emulsion nonuniformity, and misalignment of film edge to the phantom surface. For off‐axis evaluation, the dose ratio varied within 3%, and thus film is shown to be accurate for dosimetry across off‐axis distance. Conclusions: Radiographic film can be suitable for beam profile measurements and may be suitable for PDD constancy check for proton beams. Routine use will confirm such error which will be presented in this study.