Immune Cell Composition and Prognosis in Node-Positive, Irradiated Breast Cancer Patients in the DBCG-IMN2 Study

High levels of tumor-infiltrating lymphocytes (TILs) are associated with improved survival after radiotherapy in breast cancer (BC) patients, particularly in estrogen receptor (ER)-negative disease. This study investigated the prognostic relevance of immune cell subsets in irradiated patients. Node-positive patients (N = 1307) from the Danish Breast Cancer Group internal mammary node (IMN)2 study were included, in which IMN irradiation (IMNI) was allocated by laterality. Tissue microarrays were stained with multiplex immunohistochemistry for CD8+, CD4+, and FOXP3+ T-cells, CD68+ macrophages, and CD11c+ dendritic cells. Digital image analysis quantified immune infiltration across spatial compartments. Prognostic associations with distant recurrence, breast cancer-specific mortality, and overall mortality (OM) were assessed using multivariable flexible parametric survival models, including ER-stratified analyses. T-cell infiltration demonstrated a strong inverse association with OM, with 12-year OM-risk decreasing from 42% in tumors with lowest CD8+ infiltration to 27% in those with highest (HR 0.52, 95% CI (0.36-0.76)). The association was strongest in the ER-negative subgroup. In ER-positive disease, prognostic benefit was observed at low-to-moderate infiltration. CD68+ and CD11c+ infiltration correlated with improved outcomes, but with weaker effects. Immune markers were not predictive of IMNI benefit. These findings demonstrate that T-cell subsets are strong prognostic markers in irradiated BC patients, especially in ER-negative disease.