Timing of postmastectomy radiotherapy following adjuvant chemotherapy for high-risk breast cancer: A post hoc analysis of a randomised controlled clinical trial

To investigate the appropriate timing of radiotherapy (RT) after mastectomy and adjuvant chemotherapy for women with high-risk breast cancer.Post hoc analyses of 584 patients with stage II and III breast cancer from a randomised controlled clinical trial were performed. All patients underwent mastectomy followed by sequential chemotherapy and RT. The optimal cut-off values for the surgery-RT interval (SRI) and the chemotherapy-RT interval (CRI) for overall survival (OS) were determined using the hazard ratio for continuous predictors. The locoregional recurrence (LRR), distant metastasis (DM), disease-free survival (DFS), and OS rates were estimated using the Kaplan-Meier method. Multivariate analyses were performed using Cox proportional hazards regression.Median follow-up time was 83.5 months. Median SRI and CRI were 168 and 27 days, respectively. An SRI ofgt;210 days was independently associated with higher DM (HR 2.65, 95% CI: 1.49-4.71; HR 2.78, 95% CI 1.51-5.26), lower OS (HR 2.44, 95% CI: 1.28-4.54; HR 2.50, 95% CI: 1.41-4.35), and lower DFS (HR 2.57, 95% CI: 1.45-4.57; HR 2.70, 95% CI: 1.45-5.00) than SRI oflt;180 or 180-210 days. Furthermore, a CRI of more than 42 days was independently associated with higher DM (HR 1.89, 95% CI: 1.17-3.06; HR 1.96, 95% CI: 1.19-3.22), lower OS (HR 2.44, 95% CI: 1.41-4.35; HR 1.92, 95% CI: 1.10-3.33), and lower DFS (HR 1.84, 95% CI: 1.14-2.96; HR 1.82, 95% CI: 1.12-2.94) than a CRI oflt;28 or 28-42 days. However, SRI and CRI had no significant effect on LRR.Based on the present findings, the timing of the initiation of RT both after mastectomy and after the completion of adjuvant chemotherapy is crucial for patients with high-risk breast cancer.