Dosimetric Effect of Thymus on Radiation-Induced Lymphopenia in Breast Cancer Patients Who Received Adjuvant Radiotherapy

Background Radiation-induced lymphopenia (RIL), particularly severe RIL (G3+), significantly impacts cancer outcomes. This retrospective study aimed to determine the role of irradiation dose to the thymus on RIL in breast cancer (BC) patients who received adjuvant radiotherapy (RT). Methods BC patients who received adjuvant RT in 2019 and had available complete blood count data were included. RIL was defined according to CTCAE v5.0 criteria. Logistic regression identified thymic dosimetric and clinical parameters linked to G3+ RIL, ROC analysis determined optimal protective thresholds, and Delong test compared model performance. Results The data of a total of 93 consecutive patients were retrospectively retrieved, with 37.6% (35/93) experienced G3+ RIL. Logistic regression analysis identified thymic dosimetric factors (mean dose, V5, V10), baseline ALC, RT technique and IMNI were associated with G3+ RIL. ROC analysis showed that a combined model of baseline clinical and thymic dosimetry parameters had the highest AUC (0.869). The optimal cutoffs for avoiding G3+ RIL were baseline ALC ≤ 1.5 × 10 9 /L, thymus V5 ≤ 52.76%, thymus V10 ≤ 9.08%, MTD ≤ 6.12 Gy, respectively. Conclusions Thymic radiation dose correlated with severe lymphopenia in breast cancer radiotherapy. Prospective trials are needed to validate these dose constraints for clinical use.