Abstract 5959: Outcomes of small HER2+ breast cancers including those with micro-metastatic lymph node involvement (SOLEMN HER2)

Abstract Background: HER2-positive breast cancer (HER2+BC) accounts for 15-20% of early-stage breast cancers and is treated based on tumor size and Stage. For locally advanced HER2+BC [Stage (AJCC 8th ed, Anatomic) II, III], patients typically receive 2-3 chemotherapy drugs plus two HER2-targeted monoclonal antibodies (HER2-MABs): trastuzumab (T) and pertuzumab (P) before surgery. Stage I cancers are treated differently. Stage IA tumors( ≤2 cm, no lymph node involvement) usually receive one chemotherapy [paclitaxel (Tax)] plus T after surgery. However, it's unclear if chemotherapy is needed for a subset with very small tumors [T1a (≤5 mm), T1b (>5 - ≤10 mm)]. Stage IB cancers (≤2 cm tumor with micrometastases in axillary lymph node) may require more aggressive treatment. This study aims to describe various demographic, clinical, pathological characteristics, perioperative treatments, outcomes, including overall survival (OS) and invasive disease-free survival (IDFS) for patients with Stage IB (Cohort A) and very small (T1a/b) Stage IA (Cohort B) HER2+BC. Methods: This is a single institution retrospective study conducted at URMC with IRB approval. It uses electronic medical records for data collection. Patients diagnosed with Stage IB or very small (T1a/b) Stage IA HER2+BC between January 2010 - June 2024 were included in the study. Patients who had received systemic therapy before surgery were excluded. Results: Results from Cohort A are submitted in this abstract. Among seven patients included in the study, 43% had a pathological stage pT1a, 14% were pT1b and 43% were pT1c. Four patients received Tax+ T, one patient received Tax+T+P, one received two chemotherapies+T, and one received two chemotherapies+ T+P. There were no recurrence or death after a median follow-up of four years. Investigation into Stage IA cases (n= 91) is ongoing. Conclusions: This small retrospective study suggests an excellent clinical outcome for Stage IB HER2+ breast cancer patients. A larger study is needed to validate this finding. Citation Format: Uttamjot Kaur, Jacob M. Liberman, Ajay Dhakal. Outcomes of small HER2+ breast cancers including those with micro-metastatic lymph node involvement (SOLEMN HER2) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 5959.