Purpose: Radiation therapy (RT) can both suppress and stimulate the immune system. We sought to investigate the mechanisms underlying radiation-induced lymphopenia and its associations with patient outcomes in non-small cell lung cancer (NSCLC).
Methods and materials: Subjects consisted of 711 patients who had received definitive RT for NSCLC. A lymphocyte nadir was calculated as the minimum lymphocyte value measured during definitive RT. Associations between gross tumor volumes (GTVs) and lung dose-volume histogram (DVH) parameters with lymphocyte nadirs were assessed with Spearman correlation coefficients. Relationships between lymphocyte nadirs with overall survival (OS) and event free survival (EFS) were evaluated with Kaplan-Meier analysis and compared with log-rank test results. Multivariate regressions were conducted with linear and Cox regression analyses. All variables were analyzed as continuous if possible.
Results: Larger GTVs were correlated with lower lymphocyte nadirs regardless of concurrent chemotherapy receipt (with concurrent: r = -0.26, P<.0001; without: r = -0.48, P<.0001). Analyses of lung DVH parameters revealed significant correlations at lower doses (lung V5-V10: P<.0001) that incrementally decreased and became nonsignificant at higher doses (lung V60-V70: P>.05). Of note, no significant associations were detected between GTV and lung DVH parameters with total leukocyte, neutrophil, or monocyte nadirs during RT or with lymphocyte count prior to RT. Multivariate analysis revealed larger GTV (P<.0001), receipt of concurrent chemotherapy (P<.0001), twice-daily radiation fractionation (P=.02), and stage III disease (P=.05) to be associated with lower lymphocyte nadirs. On univariate analysis, patients with higher lymphocyte nadirs exhibited significantly improved OS (hazard ratio [HR] = 0.51 per 10(3) lymphocytes/μL, P=.01) and EFS (HR = 0.46 per 10(3) lymphocytes/μL, P<.0001). These differences held on multivariate analyses, controlling for common disease and treatment characteristics including GTV.
Conclusions: Lower lymphocyte nadirs during definitive RT were associated with larger GTVs and worse patient outcomes.
Copyright © 2014 Elsevier Inc. All rights reserved.