Gantry-based pencil beam scanning proton therapy for uveal melanoma: IMPT versus proton arc therapy

This study reports the single-institution clinical experience of multifield pencil beam scanning (PBS) intensity-modulated proton therapy (IMPT) and dosimetric comparison to proton arc for uveal melanoma (UM) in a regular PBS gantry room.

Eleven consecutive UM patients were treated with IMPT to 50 Gy in 5 fractions. A customized gaze-fixation device attached to the thermoplastic mask was used to reproduce the globe position for each patient. IMPT plans were robustly optimized with perturbations of 3 mm setup and 3.5% range uncertainties using 3-4 fields without apertures. Each plan was robustly reoptimized (using the same perturbation parameters) using two non-coplanar arc fields in the RayStation treatment planning system. Treatment quality for both plans was evaluated daily using CBCT-generated synthetic CT. Target coverage, conformity, and mean/maximum doses to adjacent organs were assessed.

Proton arc plans provided comparable plan quality compared to IMPT plans. Similar target coverage was achieved, with an average GTV D95% equal to 101.1% [Formula: see text] 1.0% and 101.4% [Formula: see text] 0.4% for IMPT and proton arc plans, respectively. Proton arc improves the conformity index (RTOG) compared to IMPT plans (average 0.96 [Formula: see text] 0.23 vs. 0.88 [Formula: see text] 0.18, p = 0.11). Both modalities met all the clinical goals for organs-at-risk (OARs), while proton arc significantly reduced the maximum dose for the retina from, on average, 54.5 [Formula: see text] 0.7 to 53.2 [Formula: see text] 0.3 Gy (p < 0.01). Treatment evaluation on synthetic CT showed that the doses received by patients were highly consistent with the planned doses, with a relative target coverage (D95%) difference within 3.5% for IMPT and 3.1% for proton arc, and the D95% of actual delivery exceeding 98.7% and 98.2%, respectively. The doses delivered to OARs did not exceed clinical constraints.

This is a novel report on proton arc for ocular tumors and gantry-based multifield PBS proton treatment for these tumors. This study demonstrated that both modalities can meet the clinical goals. The IMPT is currently clinically implanted, and 2-field non-coplanar proton arc plans can achieve comparable dosimetric metrics to those of IMPT plans when the deliver technique is matured.