Identifying Micropapillary Patterns With Fine-Needle Aspiration Biopsy at Metastatic Sites to Enhance Breast Cancer Diagnosis: A Case Series.

Invasive micropapillary carcinoma (IMPC) of the breast is a rare and aggressive subtype of invasive breast carcinoma, distinguished by its unique morphological characteristics. This case series analyzes fine-needle aspiration biopsy (FNAB) specimens obtained from metastatic sites in suspected breast cancer patients between 2022 and 2024, integrating detailed clinical, histopathological, and immunohistochemical information. Six samples (four axillary lymph nodes, two skin nodules) from four patients (three females, one male; mean age 60) were analyzed. Cytological examination revealed variability in cellularity and grade, as well as the presence of angulated, avascular papillaroid and morule/cell ball-like clusters with reversed cellular polarity and variable background mucin. Notably, increased cellularity, atypia, and distinct cytological patterns at metastatic sites on FNAB, coupled with reduced background mucin, showed a strong correlation with a higher proportion of IMPC in the corresponding primary breast tumors. FNAB emerges as a rapid, minimally invasive, and cost-effective technique for assessing peripheral metastases, aiding not only in the detection of this rare and aggressive subtype but also in guiding clinical management by providing valuable insight into its extent within the primary tumor.