Breast cancer (BC) patients who would benefit the most from prophylactic internal mammary node (IMN) irradiation have not been clearly identified. Risk factors for ipsilateral IMN failure in BC patients who received post-operative radiotherapy (PORT) without prophylactic IMN irradiation were investigated.

Between January 2012 and March 2013, unilateral invasive BC patients who had no clinically positive IMN and received PORT without prophylactic IMN irradiation after breast-conserving surgery or mastectomy were retrospectively reviewed. At that time, prophylactic IMN irradiation was not recommended for clinically negative IMN by Japanese BC guidelines. To identify risk factors of IMN failure, the following factors were assessed using univariate and multivariate analyses: primary tumor location, hormone receptor status, human epidermal growth factor receptor 2 (HER2), axillary node status, and administration of neoadjuvant chemotherapy (NAC).

A total of 194 BC patients (median age, 53 years; pStage I, 92; pStage II, 57; pStage III, 17; received NAC, 28) were analyzed. Median follow-up time with CT and/or FDG-PET/CT was 110 months (range, 4–132 months). IMN failure-free rates were 99% at 5 year and 98% at 10 year. The 10-year overall survival rate was 93%. In multivariate analysis, hormone receptor negativity was the only significant unfavorable factor for IMN failure (Hazard ratio 20.389, [95% CI 1.601–259.610], p = 0.02).

Hormone receptor status may have a greater impact on IMN failure compared to the primary tumor location and axillary lymph node status in the modern systemic therapy era. Because our study was a small retrospective study, further large-scale studies are needed.