Local recurrence with and without a tumour-bed boost: A post-hoc analysis of the DBCG IMN2 study

In early-stage breast cancer, a tumour-bed boost (TBB) reduces the risk of local recurrence (LR) by around 50 % but increases the risk of breast induration. LR incidences of 3 % at 5 years and 6 % at 10 years have been proposed as thresholds where benefits outweigh the potentially detrimental effects of a TBB. Therefore, this post-hoc analysis of the Danish Breast Cancer Group (DBCG) IMN2 study aimed to investigate LR rates according to prognostic risk factors to identify indications for a TBB.

From the DBCG IMN2 study, 2,430 node-positive patients operated with breast-conserving surgery were included for analysis. They received irradiation to the residual breast and regional nodes with or without internal mammary node irradiation according to laterality. Radiotherapy was 3D-conformal. TBB was delivered sequentially as 10 Gy/5 Fx (41-49 years) and 16 Gy/8 Fx (≤ 40 years or margin < 2 mm). Patients with and without a TBB were analysed separately. Prespecified subgroups included known prognostic risk factors.

Median follow-up was 13.7 years, and the cumulative incidence of LR was 1.7 % (95 % CI, 1.2-2.2) at 5 years and 3.6 % (95 % CI, 2.9-4.3) at 10 years. In patients ≥ 50 years, 1,872 patients were treated without a TBB. Among these, 145 patients with an ER-/HER2- tumour had a 10-year cumulative incidence of LR of 8.3 % (95 % CI, 4.5-13.5). No other subgroups exceeded 6 % at 10 years.

Our results suggest that node-positive patients 50 years or older with an ER-/HER2- tumour may obtain a clinically relevant benefit from a TBB.