Malignant Follicular Tumors With Myxoid Mucopolysaccharide-Like Stroma on Cytology and Histology in a Cat.

Abstract

BACKGROUND: Malignant follicular tumors are rare in cats, and cytologic description from fine needle aspirates is scarce. While a keratinaceous background is typically expected in follicular neoplasms, it may be absent in some cases, complicating cytologic interpretation. CASE PRESENTATION: A 10-year-old male Burmese cat presented with a subcutaneous abdominal mass cytologically diagnosed as a carcinoma. Approximately 2 years later, three additional subcutaneous masses developed at different locations. Cytologic evaluation of two of the most recently developed masses revealed features suggestive of a malignant melanocytic tumor with a distinctive granular background. However, histopathologic examination revealed infiltrative carcinomas with basaloid and squamous differentiation, ghost cells, keratohyalin granules, and prominent desmoplastic myxoid stroma, features supportive of a malignant follicular tumor. Immunohistochemistry demonstrated strong p63 and CK19 immunoreactivity, weak and variable cytokeratin AE1/AE3 expression, and negative CK15 labeling, supporting follicular origin. Periodic acid-Schiff (PAS) and Alcian blue stains highlighted abundant mucopolysaccharide-rich stroma. The findings were most consistent with malignant trichoepithelioma. Despite surgical excision, the cat developed additional masses and progressive clinical decline, consistent with metastatic behavior. CONCLUSIONS: This case highlights the cytologic and histologic complexity of diagnosing malignant follicular tumors in cats. The presence of abundant myxoid-like mucin-rich stroma may be associated with aggressive biological behavior. Malignant trichoepithelioma should remain a differential diagnosis for basaloid epithelial neoplasms in cats, even in the absence of cytologic evidence of keratinization. This report adds to the limited literature on feline malignant follicular tumors and emphasizes the need for further investigation into their diagnostic criteria and biological behavior.