RT-induced dynamic changes in the lymphocyte-to-monocyte ratio in patients with breast cancer indicate poor prognosis

Purpose: Radiation-induced anti-tumor responses occur in the immune system, particularly in peripheral blood mononuclear cells (PBMCs), which are overly sensitive to ionizing radiation. Irradiation of PBMCs is associated with inflammation. We assessed the association between radiotherapy (RT)-induced changes in peripheral blood cells, utilizing the lymphocyte-to-monocyte ratio (LMR), and survival outcome in breast cancer patients who underwent breast-conserving surgery followed by adjuvant RT. Methods: LMR values were obtained from retrospective data, and serial sampling of blood before the first and last RT sessions was performed. The cut-off point was analyzed using the Youden index and receiver operating characteristic curve. Recurrence-free survival (RFS) and overall survival (OS) were the main outcomes. Results: Patients with RT-induced low LMR had significantly shorter RFS (hazard ratio [HR] 2.730; 95% confidence interval [CI], 1.607–4.636, P = 0.0002) and OS (HR 2.890; 95% CI 1.410–5.924, P = 0.0038). The results were more robust in the subgroup of patients who received chemotherapy. Multivariate analysis showed that lymph node metastasis and RT-induced low LMR were associated with poor RFS (HR 1.763; 95% CI, 1.017–3.059, P = 0.044) and OS (HR 2.254; 95% CI, 1.060–4.796, P = 0.035). Conclusion: This study demonstrates that RT-induced low LMR is a valid prognostic marker for recurrence and survival in breast cancer patients undergoing RT.