PURPOSE/OBJECTIVES: Radiotherapy (RT) is curative for the majority of patients with primary indolent orbital adnexal lymphoma (IOAL). Some reports suggest inferior outcomes for bilateral IOAL, leading to uncertainty regarding optimal management for this presentation. This multicentre, retrospective study evaluated long-term outcomes of bilateral IOAL patients treated with radiotherapy alone. MATERIALS/METHODS: One hundred and eighty-four patients with synchronous (defined as ≤90 days between orbit diagnoses), or metachronous bilateral IOAL, and no prior lymphoma history, received radiotherapy to both orbits. Overall (OS), failure-free survival (FFS), freedom from local (FFLF) and distant (FFDF) failure and toxicity were evaluated. The study met local institutional review board requirements. RESULTS: A total of 184 patients were treated for IOAL from 16 centres. The median age at first orbit diagnosis was 51 years (15-91 years) with 170 (92%) of patients having MZL. Disease location included the conjunctiva in 265 (72%) of orbits. The RT dose (per orbit) was 4 Gy for 40 (11%), 20-26 Gy for 217 (59%), 27-30.6 Gy for 87 (24%), >
30.6 Gy for 24 (6.5%). Radiotherapy volume was whole orbit for 162 (44%), conjunctiva only for 177 (48%) and other partial orbit for 29 (8%). The median follow-up was 5.7 years. Ten-year OS and FFS were 99% (95% CI: 95-100) and 72% (95% CI: 62-79), respectively. Ten-year FFLF and FFDF were 87% (95% CI: 80-92) and 81% (95% CI: 72-88), respectively. Ten-year FFS was 85% (95% CI: 75-91) and 50% (95% CI: 32-65) for patients with conjunctival-only disease versus any non-conjunctival disease, respectively. Late toxicities included dry eye in 42% of patients (grade 1 in 86%) and cataract in 27%. CONCLUSIONS: In this large, multicentre experience, patients with bilateral IOAL treated with radiotherapy alone had outcomes comparable to historical series of unilateral disease. These results support the treatment of bilateral IOAL as a localised process with curative intent radiotherapy.