Patients with abscopal regressions of lymphoma occurring after palliative involved-site radiotherapy (ISRT), detected on sequential 18F-fluorodeoxyglucose-positron emission tomography (FDG-PET), were identified by an audit. A formal retrospective study was subsequently conducted to estimate the frequency of abscopal regression in follicular lymphoma (FL) after ISRT.
Potential cases were identified at multidisciplinary lymphoma meetings and were required to satisfy the following criteria: a) Palliative ISRT given for histologically-confirmed lymphoma, b) 2 or more lesions visualised on FDG-PET c) 1 or more un-irradiated lesions outside ISRT volume d) no systemic therapy delivered <2 months before RT or between RT and response assessment, e) complete metabolic response (CMR) in 1 or more un-irradiated lesions detected on serial FDG-PET/CT. All ISRT patients with FL treated 2016-2018 were systematically reviewed to provide a denominator.
Six cases were identified by audit, including 3 cases of FL. A 4th case of FL was identified by systematic review, giving 7 abscopal cases in total. In all cases a CMR was apparent both within the ISRT volume and in >1 unirradiated lesions. One patient each was identified with mantle cell lymphoma (4Gy in 2 fractions),Hodgkin lymphoma (HL) (20Gy in 3 fractions then 30Gy in 15 fractions). and Richter transformation of chronic lymphatic leukemia (30Gy in 10 fractions). Four patients with FL received either 4Gy in 2 fractions, (n=3) or 4Gy followed 8 months later by 30Gy in 15 fractions (n=1). During 2016-2018, 29 courses of ISRT were prescribed for multifocal FL, following which 4/29 (13.8%) abscopal responses were detected, including in 4/9 (44.4%) patients who had serial PET scans. Two patients, who relapsed after abscopal responses, experienced durable CMRs with immunotherapies (Immune Checkpoint Inhibitor and Donor Lymphocyte Infusion). In 4/7 cases, PET-detected abscopal regression of lymphoma occurred after 4Gy. Abscopal regressions in FL could be much more common than previously known. The combination of very low dose ISRT with immunotherapy could hold significant promise in FL.