This study aimed to evaluate dosimetrically the feasibility of saving both the lens and the ciliary body in the treatment of a choroidal melanoma by using spatially fractionated proton minibeams. A multi-slit brass collimator was designed by using Monte Carlo simulation data and was fabricated with a 0.4-mm beam opening and a 1.1-mm center-to-center distance. A phantom was also fabricated for proton beam dosimetry, involving five polymethylmethacrylate (PMMA) plates of 2 mm and 8 mm thicknesses, respectively. The peak-to-valley dose ratio (PVDR) of the proton minibeams was used to validate the feasibility of lens and ciliary body sparing. The typical single-scattered proton beam for treating a patient with a choroidal melanoma was delivered to Gafchromic EBT3 films in the phantom after having passed through the multi-slit: the energy of proton beam was 60 MeV, and its depth dose profile was a full spread-out Bragg peak (SOBP). The average dose at mid-depth of the SOBP was approximately 3 CGE with ±3% of ripple. Dosimetric evaluation showed the PVDR depended on the air gap strongly. It varied from 3.4 to 1.1 in the air gap range from 2 mm to 62 mm. The PVDR values at PMMA depths of 2, 12, 22, and 32 mm were 2.0, 1.5, 1.1, and 1.06, respectively. This result shows that a spatially fractionated proton beam might save the lens and ciliary body seated at a shallow depth in the eye ball when a choroidal melanoma is treated using proton minibeam radiotherapy.
