Pneumonitis in advanced non-small cell lung cancer: no interaction between immune checkpoint inhibition and radiation therapy.

Background: Radiation pneumonitis and immune-related pneumonitis have been studied independently, but little information has emerged on the interactions between radiation therapy (RT) and immune checkpoint inhibition (ICI). We examine whether RT and ICI are synergistic in causing pneumonitis. Methods: A retrospective cohort was assembled using the Surveillance, Epidemiology, and End Results-Medicare database, including Medicare beneficiaries diagnosed with American Joint Committee on Cancer 7th ed. (AJCC) stages IIIB-IV NSCLC between 2013-2017. Exposures to RT and ICI were determined by evaluating for treatment within 12 months of diagnosis (RT group and ICI group) and for a second exposure (e.g., ICI after RT) within 3 months after the first exposure (RT + ICI group). Untreated controls were matched to treated patients who were diagnosed in the same three-month window. A validated algorithm for identifying cases of pneumonitis in claims data was used to evaluate for the outcome within 6 months after treatment. The primary outcome was the relative excess risk due to interaction (RERI), a quantitative measure of additive interaction between two treatments. Results: There were 18,780 patients included in the analysis with 9,345 (49.8%), 7,533 (40.2%), 1,332 (7.1%), and 550 (2.9%) in the control, RT, ICI, and RT + ICI groups, respectively. Relative to controls, the hazards ratios of pneumonitis were 11.5 (95% CI: 7.9 to 17.0), 6.2 (95% CI: 3.8 to 10.3), and 10.7 (95% CI: 6.0 to 19.2) in the RT, ICI, and RT-ICI groups, respectively. The RERIs were -6.1 (95% CI: -13.1 to -0.6, P=0.97) and -4.0 (95% CI: -10.7 to 1.5, P=0.91) in the unadjusted and adjusted analyses, respectively, consistent with no evidence of additive interaction (RERI ≤0) between RT and ICI. Conclusions: In this study of Medicare beneficiaries with advanced NSCLC, RT and ICI were, at most, additive rather than synergistic in causing pneumonitis. Pneumonitis risk in patients treated with RT and ICI is not more than could be expected from each therapy alone.