Internal mammary node irradiation in 4541 node-positive breast cancer patients treated with newer systemic therapies and 3D-based radiotherapy (DBCG IMN2): a prospective, nationwide, population-based cohort study

Background: Internal mammary node irradiation (IMNI) improves overall survival (OS) in node-positive breast cancer patients. However, the effect is not documented in breast cancer patients treated with newer systemic therapies and 3D-based radiotherapy (RT). Therefore, the Danish Breast Cancer Group (DBCG) IMN2 study aimed to investigate the effect of IMNI in node-positive breast cancer patients treated with newer systemic therapies and 3D-based RT. Methods: DBCG IMN2 was a nationwide population-based cohort study prospectively allocating node-positive breast cancer patients with right-sided tumours to IMNI and patients with left-sided tumours to no IMNI in six RT centres. Exclusion criteria were prior malignancies, bilateral breast cancer, neoadjuvant systemic therapy, recurrence before RT, or non-standard RT. Systemic treatment included taxane-based chemotherapy, aromatase inhibitors, and trastuzumab. The primary end-point was OS. Secondary endpoints were breast cancer mortality and distant metastasis. Cox regression analyses were used for adjusted hazard ratios (HR). Clinicaltrial.gov ID: NCT06549920. Findings: In the period January 2007-May 2014, a total of 4541 patients were included. Patient characteristics were distributed evenly between right- and left-sided patients. Median follow-up was 13.7 years for OS. Survival rates at 15 years were 65.0% in patients with IMNI and 60.8% without leading to an adjusted HR of 0.85 (95% CI, 0.76-0.94; p = 0.0016) for OS. Corresponding HRs were 0.84 (95% CI, 0.74-0.95; p = 0.0077) for breast cancer mortality and HR 0.87 (95% CI, 0.78-0.98; p = 0.026) for distant metastasis. No subgroups were identified for the omission of IMNI. The 15-year cumulative incidence of death from ischemic or valvular heart disease was 0.2% (95% CI, 0.0-0.5) in right-sided and 0.7% (95% CI, 0.4-1.2) in left-sided patients. Interpretation: IMNI reduced distant metastasis and breast cancer mortality and improved OS in node-positive breast cancer patients, despite treatment with newer systemic therapies and 3D-based RT. Funding: This work was supported by the Danish Cancer Society and Department of Clinical Medicine, Aarhus University, Denmark.