Dosimetric feasibility of carbon-ion radiotherapy for ocular melanomas with local effect model-based treatment planning system.

This study evaluates the dosimetric feasibility of carbon-ion radiation therapy (CIRT) for ocular melanoma (OM) using a local effect model-I-based treatment planning system (TPS) combined with pencil beam scanning delivery. Cube-shaped targets were designed based on the experience of CIRT for OM at the National Institutes for Quantum Science and Technology (QST). Treatment plans used the local effect model-I-based TPS (Syngo® V13C, SIEMENS, Germany) prescribing an absorbed dose consistent with values reported in the literature. A conversion factor (CF) was established to convert the QST dose to LEM based relative biological effectiveness (RBE)-weighted dose. The physical doses were verified by an in-house Monte Carlo program and measurement. The CF for the LEM, equivalent to the QST RBE-prescribed dose of 70 Gy (RBE) over 5 fractions, was established as 0.65. The 3D gamma passing rate between TPS and MC was ≥94% using 3%-3 mm criteria. Mean dose deviations between the TPS and measurement were ≤±1.61% for all targets sizes except the smallest one (7 mm). Mean distance-to-agreement was <1 mm for all plans except one with a range shifter (distance-to-agreement = 2.92 mm). Phantom cases showed a lateral penumbra of 5.4-6.2 mm, and distal dose fall-off measures 3.6 and 5.6 mm. The results underscore the need for RBE-weighted dose conversion when applying QST's CIRT experience to OM treatment.