Purpose: To investigate the long-term impact of a boost radiation dose of 16 Gy on local control, fibrosis, and overall survival for patients with stage I and II breast cancer who underwent breast-conserving therapy.
Patients and methods: A total of 5,318 patients with microscopically complete excision followed by whole-breast irradiation of 50 Gy were randomly assigned to receive either a boost dose of 16 Gy (2,661 patients) or no boost dose (2,657 patients), with a median follow-up of 10.8 years.
Results: The median age was 55 years. Local recurrence was reported as the first treatment failure in 278 patients with no boost versus 165 patients with boost; at 10 years, the cumulative incidence of local recurrence was 10.2% versus 6.2% for the no boost and the boost group, respectively (P < .0001). The hazard ratio of local recurrence was 0.59 (0.46 to 0.76) in favor of the boost, with no statistically significant interaction per age group. The absolute risk reduction at 10 years per age group was the largest in patients <or= 40 years of age: 23.9% to 13.5% (P = .0014). As a result, the number of salvage mastectomies has been reduced by 41%. Severe fibrosis was statistically significantly increased (P < .0001) in the boost group, with a 10-year rate of 4.4% versus 1.6% in the no boost group (P < .0001). Survival at 10 years was 82% in both arms.
Conclusion: After a median follow-up period of 10.8 years, a boost dose of 16 Gy led to improved local control in all age groups, but no difference in survival.