To assess the impact of the boost dose in patients with involved surgical margins.

In the EORTC "boost versus no boost" trial, 251 patients with a microscopically incomplete tumour excision were randomised to receive either a low boost dose of 10 Gy (126 patients) or a high boost dose of 26 Gy (125 patients). Overall survival and the cumulative incidence of local recurrence as first event were compared by Logrank and Gray test, respectively (2-sided alpha=0.05), with a median follow-up of 11.3 years. The planned sample size was 660 patients, but only 251 were recruited.

The median age at randomisation was 54 years. Thirty-seven patient initially relapsed locally. At 10 years, the cumulative incidence of local recurrence was 17.5% (95% CI: 10.4-24.6%) versus 10.8% (95% CI: 5.2-16.4%) for the low and high boost dose groups, respectively (HR=0.83, 95% CI: 0.43-1.57, Gray p>0.1). Overall, 64 patients have died (25.5%), 47 of them of breast cancer, without a difference in duration of survival between the two groups (HR=0.97, 95% CI=0.59-1.5, p>0.1). Severe fibrosis was palpated in the breast in 1% versus 5% and in the boost area in 3% versus 13% in the low and high boost dose groups, respectively.

There was no statistically significant difference in local control or survival between the high boost dose of 26 Gy and the low boost dose of 10 Gy in patients with microscopically incomplete excision of early breast cancer. Fibrosis, however, was noted significantly more frequently in cases treated with the high boost dose.