Local Control after Low-Dose Radiation for Two Rare Forms of Indolent Non-Hodgkin Lymphoma.

Abstract

Small lymphocytic lymphoma (SLL) and lymphoplasmacytic lymphoma (LPL) are two rare subsets of indolent non-Hodgkin lymphoma (NHL). National guidelines recommend 24-30 Gy for localized SLL. However, based on data for follicular and marginal zone lymphoma, lower dose RT (4 Gy) has been increasingly utilized. We reviewed our experience with SLL and LPL and hypothesized that low dose RT would provide excellent local control. We retrospectively reviewed patients at three tertiary cancer centers who had been treated with RT for SLL or LPL. Response to RT was classified using the World Health Organization response criteria and by examining available PET and CT imaging. Radiographic response was assessed at first imaging follow-up and clinical response was recorded if no imaging was performed. Time to best response was noted, and Kaplan Meier estimates and cumulative incidence tests were performed to determine progression-free survival (PFS) and local progression (LP), respectively. From 2014-2022, 16 patients with 18 sites of SLL (n = 13) or LPL (n = 5) were treated with RT and available to review. Five sites of SLL represented diffuse large B-cell lymphoma transformation and were excluded from analysis. In total, eight sites of SLL (seven patients) and five sites of LPL (five patients) were treated with doses ranging from 4 to 30 Gy in 2 to 12 fractions (median 20 Gy). Four sites of disease received 4 Gy in 2 fractions, one of which (SLL) progressed approximately four months after RT. This site was successfully salvaged with 24 Gy in 12 fractions. There were no other LP. Toxicity overall was low: one patient experienced grade 2 mucositis after 25 Gy in 10 fractions to the maxillary sinus and palate and the remainder of patients experienced grade 1 or no toxicity. Of 10 symptomatic sites, seven (5/7 SLL and 2/3 LPL) attained at least partial relief after RT. A complete response (CR) was achieved in 14% of SLL disease sites and 60% of LPL sites. Partial response was achieved in 57% of SLL and 40% of LPL sites, and 29% of SLL sites were deemed to be stable. One patient with SLL died after their first RT treatment, but this was unrelated to RT. The median time to best response was 284 days (IQR 189-292 days) for SLL and 131 days (IQR 106-166 days) for LPL. 4 Gy in 2 fractions did not result in any CR, yet one patient from the LPL group exhibited a striking CR after 8 Gy in 2 fractions. PFS at one year was 51% for SLL and 100% for LPL - cumulative incidence of LP at two years was 15% and 0% respectively. In this cohort of patients with two types of indolent NHL, one patient progressed locally after 4 Gy, while none progressed after higher doses. LPL sites achieved more complete responses than SLL sites, and RT was tolerated extremely well. These results indicate that similar to other indolent lymphomas, clinical judgment should be used when deciding between 4 Gy or higher doses of RT. For SLL in particular, higher doses of RT are more likely to provide durable local control and CR.