Magnetic Resonance Imaging-Based Internal Mammary Node Response After Neoadjuvant Therapy to Guide Postoperative Internal Mammary Node Radiation Therapy Boost in Breast Cancer

PURPOSE: A radiation therapy boost to the internal mammary nodes (IMNs) is often recommended for patients with breast cancer and clinically positive IMNs at baseline, but it may increase treatment-related toxicity. Given the widespread use of neoadjuvant therapy (NAT), we investigated whether magnetic resonance imaging-assessed IMN clinical complete response (icCR) after NAT could inform response-adapted IMN boost strategies. METHODS AND MATERIALS: This retrospective study included 208 patients with clinically positive IMNs at baseline. The association between an IMN boost and outcomes was evaluated in the overall cohort and in subgroups defined by magnetic resonance imaging-assessed post-NAT icCR status. Inverse probability of treatment weighting (IPTW) was used to improve balance in selected measured baseline characteristics and reduce observed treatment selection bias. Effect modification by icCR status was assessed using IPTW-weighted Cox proportional hazards models with an interaction term. RESULTS: At a median follow-up of 36 months, an IMN boost was not associated with a significant improvement in disease-free survival (DFS) in the overall cohort (IPTW-adjusted hazard ratio [HR], 0.66; 95% CI, 0.32-1.39; P = .277). However, a significant interaction was observed between IMN boost and icCR status (P for interaction = .042). In subgroup analyses, an IMN boost was associated with improved DFS in patients without icCR (IPTW-adjusted HR, 0.36; 95% CI, 0.14-0.93; P = .034), whereas no significant improvement in DFS was observed among patients who achieved icCR (IPTW-adjusted HR, 1.13; 95% CI, 0.47-2.74; P = .781). CONCLUSIONS: In this study, patients without icCR showed improved DFS with an IMN boost, whereas those who achieved icCR showed no clear benefit. These findings support further evaluation of post-NAT icCR status as a potential indicator for selecting response-adapted postoperative IMN radiation therapy boost and warrant prospective validation.