Improving organs-at-risk sparing for choroidal melanoma patients: A CT-based two-beam strategy in ocular proton therapy with a dedicated eyeline.

Purpose: To investigate the potential clinical benefit of a two-beam arrangement technique using three-dimensional (3D) imaging of uveal melanoma (UM) patients treated with proton therapy and a dedicated eyeline.

Material/methods: Retrospective CT-based treatment plans of 39 UM patients performed using a single beam (SB) were compared to plans with two beams (TB) optimized for better trade-offs in organs-at-risk sparing. The RBE-weighted prescribed dose was 60 Gy (D = 60 Gy) in four fractions, assuming an RBE of 1.1. Dosimetric findings were analyzed for three patient groups based on tumor-optic nerve distance and UM staging (group GrA: ≤3 mm, T1 T2 UM; GrB: ≤3 mm, T3 UM; GrC: >3 mm, T1 T2 T3 UM). Finally, two schedules were compared on biologically effective dose (BED): both beams being delivered either the same day (TB), or on alternate days (TBalter).

Results: All strategies resulted in dosimetrically acceptable plans. A dose reduction to the anterior structures was achieved in 23/39 cases with the two-beam plans. D was significantly lowered compared to SB plans by 12.4 and 15.4 Gy RBE-weighted median dose in GrA and GrB, respectively. D was reduced by 18.6 and 6.0 Gy RBE-weighted median dose in GrA and GrB, respectively. A cost to the optic nerve was observed with a median difference up to 3.8 Gy RBE-weighted dose in GrB. BED differences were statistically significant for all considered parameters in favor of two beams delivered the same day.

Conclusion: A two-beam strategy appears beneficial for posterior tumors abutting the optic nerve. This strategy might have a positive impact on the risk of ocular complications.