The benefit of post-mastectomy radiotherapy in ypNO patients after neoadjuvant chemotherapy according to molecular subtypes

Purpose: The benefit of post-mastectomy radiation therapy (PMRT) in patients with breast cancer who achieve ypN0 following neoadjuvant chemotherapy (NAC) has not yet been established. This study aimed to identify the role of PMRT in patients who achieve ypN0 according to molecular subtype. Methods: We identified patients initially suspected with axillary disease who achieved ypN0 following NAC. From 13 institutions of the Korean Radiation Oncology Group between 2005 and 2011, a total of 189 patients were included in the analysis. Effects of PMRT on loco-regional control (LRC), disease-free survival (DFS), and overall survival (OS) were evaluated for different molecular subtypes. Results: In all patients, the prognostic effect of PMRT on LRC, DFS, or OS was not significant. Subgroups analysis showed that the effect of PMRT on LRC was different according to molecular subtype (p for interaction = 0.019). PMRT was associated with greater LRC in the luminal subtype (p = 0.046), but not in other subtypes. Conclusion: In patients who achieve ypN0 following NAC and mastectomy, PMRT shows no additional survival benefits for any molecular subtype.