Evaluation of Proton Arc Therapy Using a Double Scattering System with a Single, Pre-Cut Aperture with no Compensator

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To simulate and quantitatively evaluate dose from proton arc therapy using a double scattering proton therapy system with a single aperture and no compensator. Proton arc therapy was simulated with 5, 10 and 15 fields using the Mevion S250 double scattering proton therapy system and the Eclipse treatment planning system (TPS). A single aperture that provided the best conformal circular or rectangular apertures covering the tumor from the different angular views was used. No compensator was used to modulate the intensity of the fields. TPS calculated for each field the most efficient range and modulation providing the most conformal dose coverage of the target. To account for dose delivery uncertainties without a compensator, a planning-target-volume (PTV) was defined as the target. The PTV dose coverage from the arc plans were compared with the conventional proton plans prepared with 2 fields using conformal aperture and compensator for each field. Several dosimetric parameters (Dmin, Dmax, Dmean, D95, D90, D50) were used to evaluate the dose coverage of the PTV and sparing of normal tissues. The proton arc plans for spherical or rectangular regular shape targets provided superior conformal plans with lower skin and critical structure doses compared to conventional proton plans. This was due to the use of multiple beams that delivered low doses at the entrance depths and paths through normal tissues. In contrast, conventional plans with only 2 beams had relatively larger entrance and path doses, while the PTV minimum, maximum and mean doses were nearly equal in both arc and conventional plans (Table 1). The patient setup and dose delivery times were comparable with both arc and conventional plans. This has been achievable by only having to position and image the patient once for setup. Furthermore, the use of no field-specific compensator and using of one aperture size eliminated the need to enter the room and replace these devices for each field. Although, this approach of using proton arc plans had several dosimetric and delivery advantages for regular-shaped targets, the advantages has been limited for PTV with complicated shapes. Simulation of proton arc therapy shows that regular shaped targets can be treated with superior dose coverage of the PTV and sparing of critical structures using a single aperture, no compensator proton arc plans compared to conventional 2 beam proton plans with patient specific apertures and compensators. This proton arc therapy approach is superior for regular spherical or rectangular shaped targets.Abstract 3511; Table 1Comparison of different dose parameters between 2 fields conventional beam plan and proton arc beam plans simulated with 5, 10 and 15 fields with a single circular aperture and no compensatorBeamsStructureMinMaxMeanD95D90D502PTV (%)92105103100101103591106104101104109310610310110415921061041011072Lt Femur (cGy)108131692977-29123026551264712171052121410532595128768912381549248713599671303 Open table in a new tab