Single high-energy arc proton therapy with Bragg peak boost (SHARP).

INTRODUCTION: Because of the co-location of critical organs at risk, base of skull tumours require steep dose gradients to achieve the prescribed dosimetric criteria. When available, proton beam therapy (PBT) is often considered a desirable modality for these cases, but in many instances, compromises in target coverage are still required to achieve critical organ at risk (OAR) tolerance doses. A number of techniques have been proposed to further improve the penumbra of PBT. In the current study, we propose a novel, collimator-free treatment planning technique that combines high-energy shoot-through proton beams with conventional Bragg peak spot placement. The small spot size of the high-energy pencil beams provides a sharp penumbra at the target boundary, and the Bragg peak spots provide a higher linear energy transfer (LET) boost to the target centre. METHODS: Three base of skull chordoma patients were retrospectively planned with three different PBT treatment planning techniques: (1) conventional intensity-modulated proton therapy (IMPT); (2) high-energy proton arc therapy (HE-PAT); and (3) the novel technique combining HE-PAT and IMPT, referred to as single high-energy arc with Bragg peak boost (SHARP). The Monaco 6 treatment planning system was used. RESULTS: SHARP was found to improve the PBT penumbra in the plane perpendicular to the HE-PAT beams. Minimal penumbra differences were observed in the plane of the HE-PAT beams. SHARP reduced dose-averaged LET to surrounding organs at risk. CONCLUSION: A novel PBT treatment planning technique was successfully implemented. Initial results indicate the potential for SHARP to improve the penumbra of PBT treatments for base of skull tumours.

Dosimetric comparison of gantry and horizontal fixed-beam proton therapy treatment plans for base of skull chordoma.

INTRODUCTION: Australia's first proton beam therapy (PBT) centre will house a fixed-beam room and two gantry rooms. As the only PBT facility in Australia for at least the short term, there is a need to efficiently allocate treatment appointments between the gantry and fixed-beam rooms. This planning study assesses the dosimetric differences between fixed-beam and gantry-based treatment plans for base of skull chordoma, one of the core indications likely to be referred for PBT in Australia. METHODS: Retrospective gantry-based and fixed-beam treatment plans were generated for five patients with base of skull chordoma. Fixed-beam plans were generated with a conventional horizontal patient positioning system. Robust intensity modulated proton therapy (IMPT) optimisation and evaluation techniques were used for both delivery systems. Plans were designed to maximise target coverage while adhering to maximum dose constraints to neighbouring critical organs at risk. RESULTS: Robust target coverage and integral dose were found to be approximately equivalent for the gantry-based and fixed-beam plans. Doses to specific organs at risk could be reduced with the gantry-based geometry; however, the gantry-based plans did not exhibit a general decrease in doses to organs at risk. CONCLUSION: A fixed-beam treatment plan was found to be non-inferior to a gantry-based treatment plan for all base of skull patients included in the current study.