Impact of radiation dose to immune cells on survival in patients with extensive-stage small cell lung cancer receiving consolidative thoracic radiotherapy after chemoimmunotherapy.

[BACKGROUND] Consolidative thoracic radiotherapy (RT) following chemo-immunotherapy is increasingly used in extensive-stage small cell lung cancer (ES-SCLC). This study investigates the prognostic value of the estimated radiation dose to immune cells (EDRIC) and its determinants in these patients.

[METHODS] This retrospective study included 173 ES-SCLC patients between 2020 and 2023. EDRIC was calculated as a function of the number of fractions and the average doses to the lungs, heart, and remaining body. Kaplan-Meier and Cox regression analyses were performed to evaluate overall survival (OS) and progression-free survival (PFS).

[RESULTS] GTV, PTV, and N stage were positively correlated with EDRIC (r = 0.2577,  = 0.0006; r = 0.3541,  < 0.01; r = 0.2259,  = 0.0028), while lymphocyte nadir was negatively correlated (r = -0.2190,  = 0.0038). Median OS and PFS were longer in the EDRIC ≤4.68 Gy group (OS: 24.9 vs. 17.4 months,  = 0.003; PFS: 12.4 vs. 10.1 months,  = 0.038). Patients in the EDRIC ≤4.68 Gy group had significantly better OS (HR = 0.56,  = 0.003) and PFS (HR = 0.68,  = 0.039). Bone metastasis was associated with worse OS (HR = 1.88,  = 0.002), and liver metastasis with shorter PFS (HR = 2.05,  = 0.001).

[CONCLUSIONS] EDRIC is an independent predictor of OS and PFS in ES-SCLC. These findings highlight the need to optimize radiation exposure to the immune system in cancer treatment.