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Neutron Dose Equivalent Evaluation for Pencil Beam Scanning Proton Therapy with Apertures

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C Geng

C Geng1,2*, J Schuemann1, M Moteabbed1, H Paganetti1, (1) Massachusetts General Hospotal and Harvard Medical School,Boston, MA, (2) Nanjing University of Aeronautics and Astronautics, Nanjing, China


SU-E-T-567 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:To determine the neutron contamination from the aperture in pencil beam scanning during proton therapy.

Methods:A Monte Carlo based proton therapy research platform TOPAS and the UF-series hybrid pediatric phantoms were used to perform this study. First, pencil beam scanning (PBS) treatment pediatric plans with average spot size of 10 mm at iso-center were created and optimized for three patients with and without apertures. Then, the plans were imported into TOPAS. A scripting method was developed to automatically replace the patient CT with a whole body phantom positioned according to the original plan iso-center. The neutron dose equivalent was calculated using organ specific quality factors for two phantoms resembling a 4- and 14- years old patient.

Results:The neutron dose equivalent generated by the apertures in PBS is 4-10% of the total neutron dose equivalent for organs near the target, while roughly 40% for organs far from the target. Compared to the neutron dose equivalent caused by PBS without aperture, the results show that the neutron dose equivalent with aperture is reduced in the organs near the target, and moderately increased for those organs located further from the target. This is due to the reduction of the proton dose around the edge of the CTV, which causes fewer neutrons generated in the patient.

Conclusion:Clinically, for pediatric patients, one might consider adding an aperture to get a more conformal treatment plan if the spot size is too large. This work shows the somewhat surprising fact that adding an aperture for beam scanning for facilities with large spot sizes reduces instead of increases a potential neutron background in regions near target.

Funding Support, Disclosures, and Conflict of Interest: Changran Geng is supported by the Chinese Scholarship Council (CSC) and the National Natural Science Foundation of China (Grant No. 11475087).

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