Internal mammary node irradiation in breast cancer: does benefit outweigh risk?

Uncertainties around the therapeutic ratio prompted clinical trials testing regional nodal (including internal mammary node) radiotherapy, with three studies reporting in 2015.4–6 In The Lancet Oncology, Philip Poortmans and colleagues7 report 15-year results evaluating internal mammary and medial supraclavicular lymph node chain (IM-MS) irradiation, updating their 2015 publication.4 Eligible patients were aged 75 years or younger with stage I–III breast adenocarcinoma either with lateral tumours and positive axillary lymph nodes or medial or central tumours with or without positive nodes. The Danish study has the highest risk population and showed a significant improvement in overall survival and breast cancer mortality with internal mammary node radiotherapy at a median 8·9 years of follow-up; the highest absolute overall survival benefit was for patients with N2 tumours. 10-year overall survival was better than anticipated in the European Organisation for Research and Treatment of Cancer trial,7 reducing statistical power to detect a difference (74% power for HR 0·82 using observed rate in control group). The apparent discrepancy between lack of overall survival benefit despite a significant difference in breast cancer mortality might partially be explained by missing data on cause of death, which is only an issue for cause-specific survival analyses.