Effect of time interval between breast-conserving surgery and radiation therapy on ipsilateral breast recurrence

Purpose: To examine the effect of the time interval (interval) between breast-conserving surgery (BCS) and the start of radiation therapy (RT) on the subsequent risk of ipsilateral breast cancer recurrence (IBR). Methods and Materials: We reviewed interval and a number of prognostic and treatment factors among 1,962 women treated with BCS and RT for invasive breast cancer diagnosed between January 1, 1989 and December 31, 1993 in British Columbia, Canada. Subjects were female, less than 90 years old at diagnosis, not treated with chemotherapy, not stage T4 or M1, and had survived more than 30 days from diagnosis. The cumulative incidence of IBR was estimated in four interval groups: 0–5, 6–8, 9–12, and 13+ weeks. Only 23 women had an interval of greater than 20 weeks between BCS and start of RT. To assess whether an imbalance of prognostic and treatment factors could be obscuring real differences between the interval groups, Cox proportional hazards regression analyses were conducted. Results: Median follow-up was 71 months. The crude incidence of IBR for the entire sample was 3.9%. The cumulative incidence of IBR in the 6–8, 9–12, and 13+ week groups was not statistically significantly different from the cumulative incidence of IBR in the 0–5 week group. Multivariate analyses demonstrated that patients not using tamoxifen ( p = 0.027) and those with grade 3 histology ( p = 0.003) were more likely to recur in the breast. Interval between BCS and RT was not a statistically significant predictor of breast recurrence when entered into a model incorporating tamoxifen use and tumor grade (0–5 vs. 6–8 weeks, p = 0.872; 0–5 vs. 9–12 weeks, p = 0.665; 0–5 vs. 13+ weeks, p = 0.573). Conclusions: We found no univariate or multivariate difference in ipsilateral breast cancer recurrence between intervals of 0 to 20 weeks from breast conserving surgery to start of radiation therapy, in a population-based, low risk group of women not receiving adjuvant chemotherapy, after controlling for other factors important in predicting ipsilateral breast cancer recurrence.