Salvage Radiotherapy for Relapsed/Refractory Non‐Hodgkin Lymphomas Following CD19 Chimeric Antigen Receptor T-Cell (CART) Therapy

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<h3>Purpose/Objective(s)</h3> CD19-directed CART therapy has emerged as a promising treatment for relapsed/refractory non-Hodgkin lymphoma. However, sustained efficacy is limited with durable complete response rates of 40% (Schuster & Locke, 2019). Salvage radiotherapy (SRT) is potentially an important strategy post‐CART, but current data is limited to one series of 14 patients. <h3>Materials/Methods</h3> We retrospectively analyzed 21 patients who received SRT after commercial CART therapy between 8/2018 and 6/2020. Patients who relapsed after CART were divided into two groups: locoregional disease (LD, all relapsed disease encompassable within an RT field) and advanced disease (AD). SRT was defined as comprehensive (treated all sites of active disease) or focal. Post-SRT in-field objective response rates (ORR) were recorded according to Deauville or RECIST criteria with a response characterized as a complete response or partial response. Progression-free survival (PFS) and overall survival (OS) were estimated by the Kaplan-Meier method. Acute RT toxicity was graded by CTCAEv5. <h3>Results</h3> Median time from CART infusion to SRT was 4.0 months (m) (range, 0.6-11.6m). Within the LD group, 8/9 patients were treated with comprehensive SRT (median SRT dose 37.5 Gy, range 8-45 Gy, and number fractions 15, range 2-25 fractions). The in-field ORR was 8/9 (89%). For the AD group, 12/12 patients were treated with focal SRT (median SRT dose 20.8 Gy, range 8-30 Gy, and number fractions 5, range 1-12 fractions). 11/12 AD patients were treated with hypofractionated SRT (≥2.5Gy fractions). Three patients died prior to subsequent imaging with an ORR of 8/9 (89%) for the remaining evaluable patients. Distant progression occurred in 17/18 patients post-SRT (Table). Three-month PFS was 24% for the entire cohort, and 33% and 17% for the LD and AD groups, respectively (<i>P</i> = 0.258). Median OS was 7.4m for the entire cohort, and 21.1m and 2.4m for the LD and AD groups, respectively (<i>P</i> < 0.001). Grade 1 (G1) and G2 acute SRT toxicities occurred in 10 and 4 patients, respectively. No G3 or higher toxicities occurred. <h3>Conclusion</h3> SRT post-CART therapy appears safe (no ≥G3 RT toxicities) in our series. SRT led to an ORR of 89% with only 1 observed local relapse. Patients with LD relapse post-CART had significantly improved survival compared with AD relapse. Unfortunately, prognosis remains poor as 17/18 patients with evaluable imaging progressed outside of the RT field. Thus, prolonged RT courses may not be warranted given the practical inevitability of out of field disease progression. Multi-institutional review of SRT following CART is warranted to further elucidate the impact of RT dose and fractionation on disease outcomes.