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Development of a System for Evaluating the Actual Dose Distribution in Respiratory-Gated Spot-Scanning Proton Therapy Using Real-Time Image Guidance

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T Matsuura

T Matsuura1*, S Hirayama1 , H Koyano1 , S Takao1 , T Fujii1 , T Yamada1,2 , Y Fujii2 , H Nihongi2 , S Shimizu1 , K Umegaki1 , H Shirato1 , (1) Proton therapy center, Hokkaido University, Sapporo, Japan (2) Hitachi, Ltd., Ibaraki, Japan


SU-K-108-6 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 108

Purpose: To develop an in-house system for patients treated with real-time-image gated spot-scanning proton therapy (RGPT) that calculates the actual dose distribution quickly and with high accuracy.

Methods: Calculating the actual dose distribution requires the following three data items: (1) the position of the target tumor at each spot delivery timing; (2) the actual position of each spot and number of protons; and (3) the daily CT images taken immediately before treatment. Item (1) is obtained from the timing signals that control the spot delivery and fluoroscopy pulse, both of which are processed with a field-programmable gate array, as well as the positions of a fiducial marker close to the tumor, which is calculated by the gating system using fluoroscopic X-ray images. Data item (2) is recorded in the Mosaiq R&V system. After merging data items (1) and (2), a modified spot information file is created which takes into account all of the uncertainties in spot position, number of protons, and patient motion. In this study, a planning CT dataset was used for data item (3). Patient motion in RGPT is limited to ±2 mm, which allowed us to approximate it with the translational shift of a single CT dataset according to the target motion.

Results: The spot delivery and fluoroscopy pulse irradiation timings were both shown to be measured with better than 1-ms resolution. For a typical lung case (clinical target volume = 20 cc, 3 fields), the new spot information file was created within 5 s after irradiation completion and the actual dose distribution was obtained within 2 min.

Conclusion: This system has the potential to provide the actual dose distribution in a sufficiently short time to allow quick treatment replanning in day-to-day clinical conditions.

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