Abstract RF1-03: Magnetic resonance imaging based internal mammary node response after neoadjuvant therapy to guide postoperative internal mammary node boost radiotherapy in breast cancer

Abstract Background: Internal mammary node (IMN) metastasis is an adverse prognostic factor in breast cancer. While surgical dissection of IMNs is rarely performed due to its invasiveness and lack of proven survival benefit, internal mammary node irradiation (IMNI) remains the standard approach for managing clinically positive IMNs. With the increasing use of neoadjuvant therapy (NAT), a critical question emerges—whether a boost to the region of IMN metastasis is still necessary when an imaging based complete nodal response was observed. This study aims to evaluate whether omitting a postoperative IMN boost is oncologically safe in patients with baseline imaging-positive IMNs who achieve an IMN clinical complete response (icCR) following NAT. Methods: This retrospective cohort study included 1326 female breast cancer patients treated between 2018 and 2021 at our center. Inclusion criteria were: (1) histological confirmed invasive breast cancer with M0; (2) received standard NAT; (3) underwent breast-conserving or modified radical mastectomy; (4) received postoperative radiotherapy with IMNI; (5) underwent pre- and post-NAT breast magnetic resonance imaging (MRI). IMN metastasis was defined by MRI features including short-axis diameter ≥5 mm and suspicious morphological features, such as loss of hilum or high-signal in diffusion weighted imaging. IcCR was defined as complete disappearance of all previously positive IMNs on post-NAT MRI. All imaging was independently reviewed by two radiation oncologists, with discrepancies resolved by consensus. The postoperative IMNI regimen delivered 50 Gy in 25 fractions to IMN, with a subset of patients receiving a simultaneous integrated boost to 60 Gy to the pre-NAT involved internal mammary node site. Prognosis was evaluated by disease-free survival (DFS), local recurrence-free survival (LRFS), distant metastasis-free survival (DMFS), and overall survival (OS). Multivariable analysis was performed using Firth-corrected Cox regression. Sensitivity analyses including standard Cox and Bayesian Cox models with informative priors were utilized to validate the robustness of findings. Interaction between icCR status and IMN boost was also evaluated. Results: Of 1326 patients, 208 (15.7%) presented with imaging-positive IMN at baseline. The median follow-up was 53 months. IMN-positive patients had significantly worse 3-year DFS than IMN-negative patients (85.3% vs. 94.9%, HR=3.48, P