Introduction Uveal melanoma is most commonly treated with radiotherapy, destroying the tumour cells with adequate safety margins and limiting collateral damage to surrounding structures to preserve maximal vision. We used optical coherence tomography (OCT) to study the effects of radiotherapy on the retina.

Methods Patients with posteriorly located choroidal melanoma treated with proton beam radiotherapy (PBR) and ruthenium-106 brachytherapy between January 2010 and June 2014 underwent spectral domain OCT.

Results Images of 32 patients following ruthenium-106 brachytherapy and 44 patients following proton beam teletherapy were analysed. Following plaque brachytherapy, an early marked disruption of the outer retinal layers could be observed in 30 cases (94%) with retinal atrophy evident in 26 cases (81%). In contrast, the images from patients who underwent PBR showed subtle outer retinal layer change with 16 cases (36%) showing some inner-outer segment junction disruption by 6 months and 63%  by 24 months with minimal atrophy. In cases with tumours <2 mm from the fovea, the visual loss was significantly less at 6 and 12 months in the proton beam group.

Conclusion In comparison to ruthenium-106 plaque brachytherapy, PBR leads to more subtle and slower changes in the outer retinal layers enabling retention of visual function for longer. The difference in dosing regime and dose distribution across the tumour is likely to be causative for this structural differential.