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Dosimetric Impact of Beam Size for Intensity Modulated Proton Therapy in Head and Neck Cancer

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H Baba

H Baba1*, K Hotta1 , H Tachibana1 , M Kumagai1 , Y Sugama2 , T Akimoto1 , (1) National Cancer Center Hospital East, Kashiwa-shi, ,(2) Aizawa hospital, Matsumotoshi, Nagano,


SU-I-GPD-T-168 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: This study was performed to determine beam size for intensity modulated proton therapy (IMPT) in comparison to intensity modulated photon therapy (IMRT) for treating head and neck cancer.

Methods: IMPT plans were generated for 12 patients with nasopharyngeal (n=3), oropharyngeal (n=5) and hypopharyngeal (n=4) cancer with various T and N stages (T2-4N0-2cM0) received primary 9-beams IMRT, where the planning was performed using line scanning beam with the six type of beam size ranging. Proton beam size dependent in-air energy at isocenter were sigma 2.5-8 mm, 3.5-10 mm, 4-12 mm, 4.5-14 mm, 5-16 mm and 6-18 mm. In the IMPT plans, the dose was prescribed to the planning target volume (PTV) with the goal of >95% coverage by 95% of the prescription dose, and the organs-at-risk (OAR) were spared as much as possible. For each patient, dose-volume-histogram (DVH) indices including PTV coverage, homogeneity, and OAR sparing were compared IMRT plans.

Results: Increasing the beam size for IMPT plans resulted in a decrease of the target conformity and OAR sparing. For PTV coverage, the target conformity was better in IMPT plans than in IMRT plans regardless of beam size, whereas the target homogeneity was similar. OAR sparing in most IMPT plans were better than IMRT plans, whereas some IMPT plans with large beam size showed OAR sparing (Spinal cord and parotid gland) was worse. There was sharp dose fall-off between target and OAR. Limitation of the distance between target and OAR to achieve the planning prescription was dependent on beam size. The beam size less than 4-12 mm was required for IMPT plans to reduce parotid dose over IMRT plans for the cohort of patients studied.

Conclusion: We suggested a target level of beam size for head and neck cancer treated IMPT in design of proton beam delivery systems.

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