MRI-Based Internal Mammary Node Response After Neoadjuvant Therapy to Guide Postoperative Internal Mammary Node Radiotherapy Boost in Breast Cancer.

Abstract

A radiotherapy boost to the internal mammary nodes (IMNs) is often recommended for patients with breast cancer and clinically positive IMNs at baseline but may increase treatment-related toxicity. Given the widespread use of neoadjuvant therapy (NAT), we investigated whether MRI-assessed IMN clinical complete response (icCR) after NAT could help inform response-adapted IMN boost strategies. This retrospective study included 208 patients with baseline clinically positive IMNs. The association between IMN boost and outcomes was evaluated in the overall cohort and in subgroups defined by MRI-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. With a median follow-up of 36 months, the IMN boost was not associated with a significant improvement in disease-free survival (DFS) in the overall cohort (IPTW-adjusted HR = 0.66, 95% CI: 0.32-1.39, P = 0.277). However, a significant interaction was observed between IMN boost and icCR status (P for interaction = 0.042). In subgroup analyses, IMN boost was associated with improved DFS in patients without icCR (IPTW-adjusted HR = 0.36, 95% CI: 0.14-0.93, P = 0.034), whereas no significant DFS improvement was observed in patients who achieved icCR (IPTW-adjusted HR = 1.13, 95% CI: 0.47-2.74, P = 0.781). In this study, improved DFS was observed with IMN boost in patients without icCR, whereas no clear benefit was observed among those who achieved icCR. These findings support further evaluation of post-NAT icCR status as a potential indicator for response-adapted postoperative IMN radiotherapy boost selection, warranting prospective validation.