ESTRO 2025 - Abstract Book

S443

Clinical - Breast

ESTRO 2025

Details of radiotherapy planning will be provided at the conference.

Conclusion: A nodal boost was feasible and mainly delivered to the periclavicular and internal mammary lymph nodes. Despite limited follow-up, one-third of the patients experienced distant failure, most pronounced among those with HER2 normal and ER-negative/low disease. DBCG guidelines now describe nodal boost in BC.

Keywords: Breast Cancer, lymph node boost

574

Poster Discussion Radiation Therapy in Node Positive HER-2 Positive Breast Cancer: Analysis of Two Randomized Trials YOUSSEF ZEIDAN 1 , Mohammad El Jammal 2 , Omran Saifi 3 , Jose Bazan 4 , Reshma Mahtani 5 1 Radiation Oncology, Baptist Health South Florida, Boca Raton, USA. 2 Faculty of Medicine, American University of Beirut, Beirut, Lebanon. 3 Radiation Oncology, Mayo Clinic, Jacksonville, USA. 4 Radiation Oncology, City of Hope Comprehensive Cancer Center, Duarte, USA. 5 Medical Oncology, Baptist Health South Florida, Plantation, USA Purpose/Objective: The benefit of radiation therapy (RT) following modern preoperative systemic therapy (PST) in HER-2 positive breast cancer (Her2 + BC) remains unclear. The current study evaluates RT practice patterns and benefit based on the pathological nodal response to PST in axillary node-positive Her2 + BC in patients treated with breast conserving surgery (BCS). Material/Methods: TRYPHAENA and NeoSphere are two randomized phase II trials that investigated PST for Her2 + BC. The current study is a pooled analysis of both trials, focusing on 100 node-positive patients treated with HER-2 targeted PST followed byBCS. The primary goal is to describe patterns of radiotherapy practice in this population and its impact on loco-regional recurrence-free survival (LRRFS). Results: Complete nodal pathological response was achieved in 58 cases (58%). Patients with ypN0 had a 5-year LRRFS of 95.8% whereas patients with ypN+ had 5-year LRRFS of 87.4% (p = 0.105). Regional nodal radiation (RNI) was used in 16 ypN0 (47%) patients and 18 ypN+ (53%) patients (34 total patients received RNI). No significant differences in LRRFS were found for patients who received RNI vs. no RNI (93.4% vs 92.5%, P= 0.868). Distant metastasis was detected in 5 patients (5%) with the most common sites being: liver, lung, bone, and CNS. Locoregional recurrence was significantly associated with distant failure (p=0.01). Conclusion: Her2 + BC patients with initial node positive disease who achieve ypN0 after PST have excellent locoregional control. In contrast, patients with residual ypN+ trend to lower locoregional control. The utility of RNI in HER2+ BC warrants further investigation.

Keywords: HER-2 positive, neoadjuvant, recurrence patterns