To determine the interval from breast-conserving surgery (BCS) to radiation therapy (RT) that affects local control or survival.

The 10-year Kaplan-Meier (KM) local recurrence-free survival (LRFS), distant recurrence-free survival (DRFS), and breast cancer-specific survival (BCSS) were computed for 6,428 women who had T1 to 2, N0 to 1, M0 breast cancer that was diagnosed in British Columbia between 1989 and 2003, and who were treated with BCS and RT without chemotherapy. Intervals from BCS to RT were grouped by weeks as follows: < or = 4 (n = 83), greater than 4 to 8 (n = 2,288; reference group); greater than 8 to 12 (n = 2,606); greater than 12 to 16 (n = 961); greater than 16 to 20 (n = 358); and greater than 20 weeks (n = 132). Cox proportional hazards models and matching were used to control for confounding variables.

The median follow-up time was 7.5 years. The 10-year KM outcomes were as follows: LRFS, 95.4%; DRFS, 90.5%; and BCSS, 92.5%. Compared with the greater than 4 to 8 weeks group, hazard ratios (HR) were not significantly different for any outcome among patients who were treated up to 20 weeks after BCS. However, LRFS (hazard ratio [HR], 2.00; P = .15), DRFS (HR, 1.86; P = .02) and BCSS (HR, 2.15; P = .009) were inferior for women with BCS-to-RT intervals greater than 20 weeks compared with those greater than 4 to 8 weeks. The matched analysis yielded similar results.

Outcomes were statistically similar for BCS-to-RT intervals up to 20 weeks, but they were inferior for intervals beyond 20 weeks. Time can be reasonably allowed for the breast to heal and for patients to consider treatment options, but RT should start within 20 weeks of BCS.