S1765
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
Digital Poster Highlight 3984 Shoot-through enhanced proton arcs for stereotactic radiosurgery of brain metastases Johan Sundström, Erik Engwall Research and Development, RaySearch Laboratories, Stockholm, Sweden Purpose/Objective: Shoot-through proton beams have a sharper lateral penumbra compared to beams stopping in the patient. In this proof-of-concept study, we investigate how the addition of such shoot-through layers to proton arc plans can improve stereotactic treatments
of brain metastases. Material/Methods:
Three proton arc techniques were compared to a VMAT baseline: Bragg peak layers only (BP), shoot- through layers only (ST), and the combination (BP+ST) [1]. The proton plans were planned for an IBA ProteusPlus, and the VMAT baselines were planned for a 6 MV FFF Varian TrueBeam STx using a target partitioning approach [2]. All plans used arc trajectories at couch angles: 0° (full arcs), 30°, 330° (half arcs). The four planning techniques (VMAT, BP, ST, BP+ST) were first evaluated on a series of simulated single-lesion cases with a spherical, centrally located GTV of varying size (radius range 1 to 14 mm) and a 1x21 Gy prescription. Photon plans used a 1 mm PTV margin, whereas proton plans were robustly optimized for GTV prescription dose coverage in 21 scenarios (3% range errors, 1 mm setup errors), and additionally for 120-140% dose within the GTV in the nominal-scenario to mimic the spatial dose distribution of the photon plans.The planning techniques were further evaluated on the six clinical multi-lesion cases described in [2], reusing the OAR objectives from the reference but adapting the target objectives to match the single-lesion objectives described above. All plans (photon and proton) were robustly evaluated over 16 scenarios (±3% range errors, 1 mm diagonal setup errors). Results: Figure 1 illustrates that ST behaved like VMAT although with a less sharp fall-off near the smallest targets. BP resulted in a worse fall-off near small targets, but unlike for VMAT and ST, the shape of the fall-off remained relatively constant as a function of target size. BP+ST combined the benefits of both techniques, the BP fraction increasing with target size.
For the multi-lesion cases, all plans achieved adequate robust GTV coverage, and fraction delivery times were similar across techniques. BP failed to outperform VMAT except in the low-dose region, whereas BPP+ST on average achieved similar conformity index (PCI), gradient index (GI) as VMAT while considerably reducing isodose volumes for dose levels below 50% of the prescription.
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