The Impact of Radiation Therapy on Lymphocyte Count and Survival in Metastatic Cancer Patients Receiving PD-1 Immune Checkpoint Inhibitors.

Purpose: Therapeutic radiation has conflicting immune effects: radiation therapy (RT)-induced immunogenic cell death can contribute to immune response, but lymphocytes are also sensitive to RT. It is unknown whether palliative RT leads to lymphopenia in patients treated with immune checkpoint inhibitors (ICIs) and whether this affects outcomes. As such, we sought to assess the impact of palliative RT on circulating lymphocyte count and neutrophil-to-lymphocyte ratio in patients being treated with PD-1-directed ICI and associations with survival.
Methods and Materials: We identified patients from 5 radiation oncology centers, treated with palliative RT and either pembrolizumab or nivolumab with non-small cell lung cancer, metastatic melanoma, and renal cell carcinoma. Patients who received intervening cytotoxic chemotherapy were excluded. We recorded absolute lymphocyte count (ALC) and neutrophil-to-lymphocyte ratio before and after palliative RT and at the start of ICI. Survival was analyzed using the Kaplan-Meier method and Cox proportional hazard models.
Results: One hundred ten patients received 225 courses of palliative RT. Median change in ALC after RT was -161 cells/mL. Decreases in ALC were greater with RT to the spine, lung/mediastinum, and chest wall compared with the brain, extremity, or abdomen/pelvis (P = .002) and after courses >5 fractions (P = .003). Extracranial and >5-fraction RT was associated with increased odds of severe lymphopenia (ALC <500) at the end of RT (odds ratio [OR], 3.7; P = .001; and OR, 3.9; P = .001, respectively). Patients who developed RT-induced severe lymphopenia were more likely to have severe lymphopenia when ICI was initiated (OR, 6.4; P = .0001), particularly when RT was administered in the previous 3 months (OR, 189; P < .0001). Severe lymphopenia at onset of ICI therapy was associated with increased mortality on multivariable analysis (hazard ratio, 2.1; P = .03).
Conclusions: Extracranial or prolonged courses of RT increase the risk of severe lymphopenia, which is associated with poorer survival in patients treated with ICI.
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