Purpose: The significance of radiation dose to the host immune system during the treatment of stage III non-small cell lung cancer (NSCLC) is unknown, but higher doses were associated with worse tumor control and overall survival (OS) in a secondary analysis of RTOG 0617. In this study, we sought to assess the impact of the estimated dose of radiation to immune cells (EDRIC) on cancer-specific outcomes in an independent cohort of patients treated at our institution.
Methods and materials: We retrospectively identified 117 patients with stage III NSCLC treated with definitive fractionated radiation from 2004 to 2017 at a single academic center (median dose of 60 Gy; 60% underwent intensity modulated radiation therapy and 92% received concurrent platinum-based chemotherapy). EDRIC was calculated as a function of the number of radiation fractions and mean doses to the lung, heart, and remaining body based on a model developed by Jin et al.
Results: Median follow-up was 16 months with 77% of patients followed until death. In the entire population, 5-year OS was 11.2% with a median survival of 17.3 months. Median EDRIC for the entire cohort was 6.1 Gy (range, 2.5-10.0 Gy). A higher EDRIC was correlated with greater risk of grade ≥3 lymphopenia (P = .004). On multivariate analysis including total prescription radiation dose, planning target volume, and chemotherapy utilization, EDRIC was independently associated with OS (hazard ratio [HR] 1.17, P = .03), local progression-free survival (HR 1.17, P = .02), and disease-free survival (HR 1.15, P = .04). The median OS for patients with an EDRIC above 7.3 Gy (fourth quartile) and below 5.1 Gy (first quartile) was 14.3 and 28.2 months, respectively.
Conclusions: Higher doses of radiation to the immune system were associated with tumor progression and death after the definitive treatment of stage III NSCLC. Tailoring radiation therapy to spare the immune system may be an important future direction to improve outcomes in this population.
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