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Effectiveness of An In-Gate Beam Tracking Method in Spot-Scanning Proton Therapy

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T Matsuura

T Matsuura1*, Y Fujii2, R Fujimoto2, M Umezawa2,N Miyamoto1, K Sutherland1, S Takao1, H Nihongi1, C Toramatsu1, S Shimizu1, K Umegaki1, H Shirato1, (1) Hokkaido University Graduate School of Medicine, Sapporo, Hokkaido,(2) Hitachi Ltd., Hitachi Research Laboratory, Ibaraki

SU-E-T-448 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To mitigate the impact of tumor motion in spot-scanning proton beam therapy, the gating technique has been considered as a powerful tool. Although residual tumor motion of a few millimeters of gate volume can still cause a dose error, the error can be minimized by repainting. However, a larger number of paintings also results in the prolongation of treatment time. We propose a method which improves the dose uniformity while minimally increasing the number of paintings.

Methods: Lateral beam tracking was applied in conjunction with gating. A simulation study was performed using the VQA treatment planning system (Hitachi Ltd., Japan) to assess the effectiveness of this method. A gated proton beam provided by a synchrotron was irradiated to spherical clinical target volumes (CTVs) with a diameter of 5 cm located at 10 cm and 15 cm depth in a heterogeneous phantom. The system delay time was 66 ms. Gate width was set to 2 mm. Rigid motion was assumed for CTVs which were shifted in synchronization with 20 patients' tumor trajectory data. A dose of 2 Gy was prescribed with the minimum repainting scheme. The cases that fulfilled the ICRU 50 criteria (CTVmax<107% and CTVmin>95%) were accepted.

Results: When only gating was used, 12 and 16 out of 20 cases were accepted for targets at 10 cm and 15 cm depth, respectively. On the other hand, when beam tracking was also applied, all cases were accepted for both target depths.

Conclusion: The results suggest that the application of lateral beam tracking in gate volume can improve the dose uniformity without (or minimally) increasing the number of paintings. A simulation including non-rigid patient motion will be considered in the future.

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