Postmastectomy radiotherapy (RT) in high-risk breast cancer patients can reduce locoregional recurrences (LRRs) and improve disease-free and overall survival. The aim of this analysis was to examine the overall disease recurrence pattern among patients randomly assigned to receive treatment with or without RT.

A long-term follow-up was performed among the 3,083 patients from the Danish Breast Cancer Cooperative Group 82 b and c trials, except in those already recorded with distant metastases (DM) or contralateral breast cancer (CBC). The end points were LRR, DM, and CBC, and the follow-up continued until DM, CBC, emigration, or death. Information was selected from medical records, general practitioners, and the National Causes of Death Registry. The median potential follow-up time was 18 years.

The 18-year probability of any first breast cancer event was 73% and 59% (P < .001) after no RT and RT, respectively (relative risk [RR], 0.68; 95% CI, 0.63 to 0.75). The 18-year probability of LRR (with or without DM) was 49% and 14% (P < .001) after no RT and RT, respectively (RR, 0.23; 95% CI, 0.19 to 0.27). The 18-year probability of DM subsequent to LRR was 35% and 6% (P < .001) after no RT and RT, respectively (RR, 0.15; 95% CI, 0.11 to 0.20), whereas the probability of any DM was 64% and 53% (P < .001) after no RT versus RT, respectively (RR, 0.78; 95% CI, 0.71 to 0.86).

Postmastectomy RT changes the disease recurrence pattern in high-risk breast cancer patients; fewer patients have LRR as first site of recurrence, and overall fewer patients have DM.