High Local Control and Low Ocular Toxicity Using Ultra-Low-Dose “Boom-Boom” Radiotherapy for Indolent Orbital Lymphoma

Background: The first line definitive treatment for early-stage indolent B-cell lymphoma is radiation therapy (RT). Due to the sensitivity of orbital structures to radiation, ultra-low-dose RT (4 Gy in 2 fractions, "boom-boom") has and been utilized as an attractive option for orbital lymphoma. In this retrospective study, we evaluated the outcome and toxicity of "boom-boom" RT for indolent orbital lymphoma with an emphasis on ophthalmologic toxicity. Methods: This is a retrospective case series with 17 patients with orbital lymphoma who received boom-boom RT at a single tertiary referral center between January 2017 and June 2022. Medical records, imaging and radiation treatment plans were reviewed. Endpoints included response rate, progression, and ocular toxicity per oncologist and ophthalmology reports. Results: A total of 17 patients (12 female and 5 male) with 19 indolent orbital lymphomas were included. Median follow-up was 39 months. Complete, partial, and stable response was achieved in 65%, 24%, and 12% of patients, respectively. Only 1 patient developed local recurrent 47 month after radiation treatment, and was successfully salvaged with standard dose radiation (24 Gy). Five-year distant progression rate is 18%. Oncologist-reported Common Terminology Criteria for Adverse Events (CTCAE) toxicity rates were 6% grade 1 and 0% grade 2+. Ophthalmologist reported 33.3% new post-RT toxicities including dry eye, cataract, and chorioretinal atrophy. There is no significant vision acuity change after RT. Conclusions: "Boom-Boom" RT (4 Gy in 2 fractions) provides excellent control for indolent orbital lymphoma. While minimal toxicity was documented by radiation oncologists, higher rates were noted by ophthalmologists, highlighting the radiosensitivity of orbital structures and potentially underreported ocular toxicity in "boom-boom" and standard regimens. Further prospective randomized studies are needed to better define the outcome and toxicity of ultra-low-dose (4