ESTRO 2024 - Abstract Book

S729

Clinical - Breast

ESTRO 2024

University Hospital, Department of Oncology, Aalborg, Denmark. 7 Odense University Hospital, Department of Oncology, Odense, Denmark. 8 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark

Purpose/Objective:

Internal mammary node irradiation (IMNI) reduces distant recurrences and breast cancer (BC) mortality and improves overall survival (OS) in high-risk BC patients. However, it is unclear if the effect remains in the landscape of screening, 3D-based radiotherapy (RT), and modern systemic therapies, thereby still outweighing the potential risk of long-term heart toxicities. Therefore, the Danish Breast Cancer Group (DBCG) IMN2 study aims to investigate the effect of IMNI in the era of modern adjuvant therapies.

Material/Methods:

A nationwide prospective cohort study was conducted from 2007 to May 2014. IMNI was indicated in right-sided patients but not in left-sided. Inclusion criteria were node-positive BC patients treated with loco-regional RT. Exclusion criteria were prior malignancies, bilateral BC, primary systemic therapy, recurrence before RT, and non standard RT. IMNI technique was 3D-based RT with tangential fields in free-breathing. Systemic adjuvant treatment was taxan-based chemotherapy, tamoxifen/letrozole for pre-/postmenopausal patients, and trastuzumab for HER2 positive patients. Data were collected from the DBCG database and the Danish Pathology Data Bank. Missing values and inconsistencies were handled with chart reviews. The primary end-point was overall survival (OS). Secondary endpoints were BC mortality and distant recurrence. Cox regression analyses were used for crude and adjusted hazard ratios (HR). The Kaplan-Meier and the Aalen-Johansen estimators were used for survival and cumulative incidence curves. For secondary end-points, non-BC death was a competing event. End of follow-up was the 30th of November 2023. Of 5206 eligible patients, 4541 patients were included. Patient characteristics were distributed evenly between right and left-sided patients, table 1. Patients were treated per protocol, thus right-sided patients received IMNI, left sided did not (abstract E24-761). Median follow-up was 13.6 years for OS. Survival rates at 15 years were 65.1% in patients with IMNI and 61.0% without leading to a crude HR for OS of 0.86 (95% CI, 0.77-0.95; p=0.003) and an adjusted HR of 0.85 (95% CI, 0.77-0.94; p=0.002). The 15-year cumulative incidence of BC mortality was 22.5% in patients with IMNI and 25.0% without. Crude HR for BC mortality was 0.86 (95% CI, 0.76-0.98; p=0.024), and adjusted HR was 0.84 (95% CI, 0.74-0.96; p=0.009). The 15-year cumulative incidence of distant recurrence was 25.0% in patients with IMNI and 26.7% without. Crude HR for distant recurrence was 0.92 (95% CI, 0.82-1.03; p=0.143), and adjusted HR was 0.89 (95% CI, 0.79-0.1.00; p=0.043). Thus, all cancer specific endpoints were in favor of IMNI. The 15-year cumulative incidence of death from ischemic or valvular heart disease was 0.24% in right-sided patients and 0.8% in left-sided. Results: