Purpose: To investigate the impact of the boost dose to the primary tumour bed in the framework of breast conserving therapy on local control, cosmetic results, fibrosis and overall survival for patients with early stage breast cancer.
Patients and methods: Five thousand five hundred and sixty-nine patients after lumpectomy followed by whole breast irradiation of 50 Gy were randomised. After a microscopically complete lumpectomy (5318 patients), the boost doses were either 0 or 16 Gy, while after a microscopically incomplete (251 patients) lumpectomy randomisation was between 10 and 26 Gy. The results at a median follow-up of 10 years are presented.
Results: At 10 years, the cumulative incidence of local recurrence was 10.2% versus 6.2% for the 0 Gy and the 16 Gy boost groups (p < 0.0001) and 17.5% versus 10.8% for the 10 and 26 Gy boost groups, respectively (p > 0.1). There was no statistically significant interaction per age group but recurrences tended to occur earlier in younger patients. As younger patients had a higher cumulative risk of local relapse by year 10, the magnitude of the absolute 10-year risk reduction achieved with the boost decreased with increasing age. Development of fibrosis was significantly dependent on the boost dose with a 10-year rate for severe fibrosis of 1.6% after 0 Gy, 3.3% after 10 Gy, 4.4% after 16 Gy and 14.4% after 26 Gy, respectively.
Conclusion: An increase of the dose with 16 Gy improved local control for patients after a complete lumpectomy only. The development of fibrosis was clearly dose dependent. With 10 years median follow-up, no impact of survival was observed.