Background/Aims A retrospective service evaluation to report on recurrence of iris melanoma after proton beam therapy (PBT).

Methods The cohort comprised 150 consecutive eyes with iris melanoma treated with proton beam radiotherapy. Treatment consisted of 53.1 Gy of proton beam irradiation delivered in four fractions over four consecutive days. Main outcome measures were local tumour control and tumour-related mortality.

Results A total of 150 patients were treated. The median follow-up time was 66 months (range, 12–108 months). The tumour ultrasound thickness ranged from 1.1 mm to 4.9 mm, with a median of 2.4 mm. The tumours in these patients had a median largest basal diameter of 6.7 mm (range, 2.9–8.4 mm), involving more than two clock hours of iris or angle in five patients. The ciliary body was involved in three patients. Eight patients (5.3%) presented with local recurrence, diagnosed after a median follow-up of 24 months (range, 12–84 months). The recurrence was focal in two patients and diffuse in six. Six eyes were enucleated whereas one eye underwent iridocyclectomy and one received a second course of PBT.

Conclusions PBT provides excellent local tumour control and ocular preservation with good retention of vision. Most recurrences occur because the extent of the tumour is clinically underestimated but some tumours are radioresistant. Recurrence can occur several years after treatment. These findings indicate the need for wide safety margins and even radiotherapy of the entire anterior segment in patients with diffuse disease. Surveillance after treatment needs to be lifelong.