SU‐C‐105‐04: Measurement of Absorbed Dose Due to Secondary Radiation Produced by Passively Scattered and Uniformly Scanned Proton Beams

Abstract

Purpose: Using the dual ionization chamber (IC) method we compare the neutron and combined gamma + proton (γ+p) components of absorbed dose in water due to secondary radiation produced by passively scattered and uniformly scanned proton beams collimated by a tungsten MLC. Methods: We adapt the dual IC method of mixed field dosimetry for use in measuring the components of absorbed dose due to secondary radiation produced by proton beams. Measurements are performed in water 5 cm beyond the distal edge and 15 cm lateral to the beam isocenter for passively scattered/uniformly scanned proton beams. Beams with ranges of 15 cm or 25 cm and modulation 10 cm (R15M10 and R25M10) are employed, collimated to 10 × 10 cm field size at isocenter with the MLC. Results: Measured neutron dose per primary proton dose ranges from 0.2–0.4 mGy/Gy for R15M10 and 0.3–0.8 mGy/Gy for R25M10 beams. The secondary γ+p dose is 0.3–0.7 mGy/Gy for R15M10 and 0.5–2.0 mGy/Gy for R25M10. Doses are reported here as physical dose without application of a radiation quality factor. Conclusion: The largest difference in secondary radiation dose for the two modalities is seen at 5 cm beyond the distal edge for R15M10, with increases of 40–50% for neutron dose and 20% for γ+p for scattered over scanned beams. For the R25M10 beam the difference is smaller, about 10% higher neutron and γ+p doses for scattered over scanned beams. Outside the beam at 15 cm lateral to isocenter, the γ+p dose dominates the neutron dose and smaller differences are observed between the two beam delivery modalities. This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17‐W81XWH‐07‐2‐0121. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the US Army. This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17‐W81XWH‐07‐2‐0121. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the US Army.