Surface as a motion surrogate for gated re-scanned pencil beam proton therapy.

This simulation study investigated the dosimetric effectiveness and treatment efficiency of surface motion guided gating of pencil beam scanning (PBS) proton therapy for liver tumour treatments. Dedicated 4D dose calculations were performed for simulating gated treatments using 4DCT data for six patients derived from 4DMRI (4DCT(MRI)). Surface motion as a surrogate for tumour motion was extracted from the 4DMRI images and a linear internal-external correlation model applied to derive amplitude-based gating windows (GWs) of 10 and 5 mm. 4D treatments were simulated using gating and layered/volumetric rescanning (either alone or combined) and four assumed system latencies (50/100/200/500 ms) for the response time of the beam gating to the surrogate. Resulting 4D plans were compared using D5-D95 and V95 in the CTV as the primary metrics, as well as dose to the healthy liver and total treatment time. With no motion mitigation, interplay effects deteriorate the dose homogeneity by more than 30% with respect to the static reference plan, whereas with surface motion guided gating alone, this could be reduced to 12/20% and 5/10% (mean/max over all cases) for 10 mm and 5 mm GWs, respectively. Furthermore, by combining  ×5 layered rescans with 5 mm GW, plan homogeneities to within 1/5% of the static references could be achieved. Dose inhomogeneities were however still pronounced for latencies  ⩾200 ms but limited when  ⩽100 ms. ITV volumes could be decreased by 19/25% when 10/5 mm GW was employed, leading to reductions in mean dose to the healthy liver tissue of 2.6/3.3%. Our results confirm the potential of combining gating and re-scanning (re-gating) for mitigating large tumor motions, and the potential of surface motion monitoring as a gating signal.