Priming radioimmunotherapy with external beam radiation in patients with relapsed low grade non-Hodgkin lymphoma

Background: The aim of this study was to evaluate the outcomes of priming salvage radioimmunotherapy (RIT) with a low dose of external beam radiotherapy (EBRT) in patients with relapsed low grade non-Hodgkin lymphoma (LG-NHL). Methods: Patients who received salvage RIT with or without 2 x 2 Gy EBRT between March 2009 and February 2013 were retrospectively reviewed at a single institution. Planning target volume (PTV) for EBRT was created by adding a 1-2 cm expansion to the gross tumor volume depending on the anatomical location. Kaplan-Meier method via log-rank was employed to analyze the endpoints freedom from progression (FFP) and overall survival (OS). Results: We identified 22 patients who received salvage RIT without chemotherapy with a median follow up of 34 months. Of these, 9 (41%) patients were treated with EBRT immediately prior to RIT, and 13 (59%) received salvage RIT alone. Median FFP was not reached in patients who underwent combination treatment, while it was 9 months for patients treated with RIT alone (p = 0.02). OS for all patients at 36 months was 80.3% with no significant difference between the two groups (p = 0.88). On univariate analysis, the addition of EBRT was associated with improved FFP [hazard ratio (HR) = 4.17; 95% confidence interval (CI), 1.24-19.1; p = 0.02)]. No long term toxicities were reported in both groups. Conclusions: RIT outcomes and effects were improved with addition of low-dose EBRT immediately prior to it, in the treatment of relapsed LG-NHL with no additional toxicity. This study is hypothesis-generating and the findings should be validated in prospective studies.