Radiotherapy for indolent primary cutaneous B-cell lymphoma: an international multicenter ILROG analysis.

Radiotherapy is an established treatment for low-grade primary cutaneous B-cell lymphoma. Recommendations on its use differ internationally, which prompted our group to conduct the present analysis. Twenty-two institutions participated in this international study. Patient eligibility required a diagnosis of limited (T1/T2) primary cutaneous marginal zone or follicle center lymphoma treated with radiotherapy between 1995 and 2023. Data were collected retrospectively until February 2024 in the framework of the International Lymphoma Radiation Oncology Group. Overall, 535 patients were analyzed. Predominant locations were the head (40%) and trunk (36%). Radiotherapy had a median dose of 24 Gy in fractions of 2 Gy. Complete responses were seen in 91% at a median time of 3.6 months following radiotherapy. There was no statistically significant difference between treatments ≤4 Gy or >4 Gy for complete or overall response rates (p=0.077 and p=0.056). However, there was an inferior duration of local control with ≤4 Gy (5-year local control 73% ± 12% vs. 96% ± 2%; p<0.001). Radiation dose was the main prognostic factor in the univariate and multivariate Cox analysis; however, higher doses did not translate into an overall survival benefit. Toxicities rarely exceeded grade 2 but were more frequent in the >4 Gy group. Radiotherapy remains an effective treatment option for indolent skin lymphoma with low toxicities. High response rates are observed with low doses ≤4 Gy. In comparison to conventional doses, these treatments have a shorter duration of local control but a favorable toxicity profile.