SU-FF-T-367: Radiochromic Film and Ion Chamber Dosimetry for Monochromatic X-Rays in PMMA

Abstract

Purpose: K‐edge capture radiotherapy using monochromatic, keV x‐ray beams necessitates accompanying dosimetry methods. This work compares radiochromic film and ion chamberdosimetry methods potentially suitable for use with monochromatic x‐ray beams. Method and Materials: X‐rays were produced at the LSU CAMD synchrotron by passing a 1.3‐GeV electron beam (≈200‐mA) through a 7‐T superconductingwiggler. The resulting polychromatic beam was passed through a double multilayermonochromator to generate an approximately 0.1×2.8‐cm2, 35‐keV x‐ray beam. A 2.5×2.8‐cm2 broad beam was produced via oscillation of phantom and dosimeters by a triangular waveform. Central‐axis depth dose was measured in a 10×10×12.5‐cm3 PMMA slab phantom using 5.12×5.12‐cm2 GAFChromic® EBT films and an air‐equivalent, cylindrical ion chamber (0.23‐cm3). Films were digitized using the red channel of a flatbed scanner, and pixel values were converted to dose using both 6‐MV x‐ray and 125 I brachytherapy seed calibration curves. 125 I doses were calculated using AAPM TG‐43 formalism. Ion chamber charge readings were converted to dose using the AAPM TG‐61 protocol for kilovoltage x‐ray beam dosimetry.Results: Measurements in a PMMA phantom yielded film depth‐dose curves from film that were 2.5–4.4% higher than those from the ion chamber for depths of 0 to 9 cm when using the 125 I seed calibration. Using the 6‐MV x‐ray dosecalibration for film resulted in doses approximately 35% lower due to a significantly different film calibration curve compared to that using 125 I seeds. Conclusion: These methods should be suitable for future dose measurements required for cell and small animal irradiations. The discrepancy between 6‐MV x‐rays and 125 I seeds is contrary to previously reported results and currently under investigation.