Pathological Complete Response to Neoadjuvant Anthracycline–Taxane Chemotherapy in Stage IIIB Luminal B Her2-Negative Invasive Micropapillary Carcinoma of The Breast: A Case Report

Invasive micropapillary carcinoma (IMPC) of the breast is a rare and aggressive histological subtype, accounting for approximately 1–2% of breast cancers and characterized by a high propensity for lymphovascular invasion and lymph node metastasis. Among these cases, fewer than 10% achieve a complete clinical response following neoadjuvant chemotherapy. This case report aims to provide a comprehensive clinical and pathological description of a rare case of Stage IIIB Luminal B HER2-negative invasive micropapillary carcinoma that achieved a pathological complete response (pCR) following neoadjuvant anthracycline–taxane chemotherapy and to analyze the clinicopathological and molecular factors contributing to this exceptional treatment response. This research presents a case-based analysis of a 44-year-old female patient diagnosed with locally advanced breast cancer. Clinical evaluation, imaging examinations (breast ultrasound, chest X-ray, and liver ultrasound), histopathological assessment, and immunohistochemical analysis (ER, PR, HER2, and Ki-67) were performed to establish the diagnosis, staging, and molecular subtype. Tumor growth kinetics were also estimated using tumor doubling time. The patient presented with a progressively enlarging breast mass accompanied by peau d’orange appearance and axillary lymphadenopathy. Histopathological examination confirmed grade 3 IMPC, while immunohistochemistry revealed a Luminal B HER2-negative subtype (ER/PR-positive, Ki-67: 40%). Imaging examinations showed no evidence of distant metastasis (T4bN1M0, Stage IIIB). Following AC-T neoadjuvant chemotherapy, a complete clinical response was observed, with the disappearance of the primary tumor and axillary lymph node involvement. IMPC exhibits aggressive local behavior with a high rate of nodal involvement; however, it may respond favorably to systemic therapy when appropriately managed through multimodal evaluation, accurate molecular characterization, and early detection.