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A New SOBP-Formation Method by Superposing Specially Designed Composite Bragg Curves (CBCs) for Spot Scanning Proton Beam Therapy

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K YOKOKAWA*, K Umegaki , T Matsuura , S Hirayama , M Furusaka , Hokkaido University, Sapporo, Hokkaido


SU-H2-GePD-T-5 (Sunday, July 30, 2017) 3:30 PM - 4:00 PM Room: Therapy ePoster Lounge

Purpose: The main purpose of this study is to propose a new SOBP-formation method for low energy beam region in spot scanning proton beam therapy in order to solve the following conflicting problems; (1) keep the high dose uniformity, (2) reduce required number of energy layers, (3) keep distal-falloff as sharp as possible, (4) mitigate dose error caused by range uncertainties and (5) keep simple irradiation system to install any facilities.

Methods: We use only one specially shaped mini-ridge filter (MRF) to create the new Composite Bragg Curves (CBCs) from very sharp pristine Bragg curves of low energy proton beams. The CBC has three pre-designed dose regions of front increasing, flat-top and distal falloff. These three regions are formed as to have almost equal depth lengths and good linearity. Then a required SOBP can be formed by superposing several CBCs with correct spacing and beam intensity weight. In order to verify the effectiveness of the proposed method, SOBP performances of (1) to (4) have been compared with conventional SOBP formed by pristine Bragg curves.

Results: The dose uniformity of the SOBP formed by the proposed method is within ±1% in various region lengths of the CBCs and is almost similar to that of conventional SOBP. The required number of energy layers is reduced to less than one-third of conventional SOBP. The sharpness of distal falloff can be controlled by appropriate design of the region length and kept equal to that of conventional SOBP. The dose error caused by range uncertainties is sufficiently mitigated, especially at the deepest energy layer.

Conclusion: The proposed method has a potential to solve the conflicting problems of SOBP-formation for low-energy beam region in spot scanning proton beam therapy. This irradiation system can be installed in any facilities with only one specially shaped MRF.

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