Cytology of axillary breast carcinoma metastasis with squamous features - a morphological spectrum of diagnostically problematic aspirates.

Background Fine-needle aspiration cytology of axillary lymph nodes plays an established role in the staging and management of breast cancer. Squamous differentiation in breast carcinomas can be seen in metaplastic carcinomas, primary squamous cell carcinomas and treatment effects. This study reviews a cohort of metastatic breast carcinomas with squamous features in axillary lymph node aspirates. Methods A computerized search of the department pathology archives from 2016 to 2026 was performed for breast specimens with histologic evidence of squamous features matched to axillary lymph node aspirates, and malignant axillary lymph node aspirates with cytologic evidence of squamous features. Slides, reports and clinical information were reviewed. Results A total of 13 axillary lymph node aspirates were retrieved from 12 patients aged 42 to 79 years. Cellularity was variable and glandular clusters were seen in only 1 case. Keratin pearls were present in 6 cases. The majority of histologic correlates were invasive ductal carcinomas (n = 8/12), with histologic squamous differentiation in 6. There were 2 metaplastic carcinomas, 1 mixed ductal/lobular carcinoma with squamous differentiation, and 1 case with complete pathological response. Of 10 cases with biomarker status, 5 were triple-negative, 4 hormone-positive and 1 HER2-overexpressed. Conclusion Squamous morphology in axillary node aspirates should raise suspicion for metastatic breast carcinoma with squamous differentiation. This scenario is prone to gaps across clinical, histological, cytological, and immunocytochemical layers. Careful observation of subtle glandular components, accompanied by review of histological findings and immunocytochemistry, is important for accurate diagnosis.