Fibromyxoid Nephrogenic Adenoma Mimicking Renal Cell Carcinoma: A Diagnostic Pitfall in Fine-Needle Aspiration Cytology.

Nephrogenic adenoma (NA) is a benign lesion of the genitourinary tract that may mimic malignancy both clinically and histologically. The fibromyxoid variant (FMNA), characterized by spindled cells in myxoid stroma, presents additional diagnostic challenges due to its rarity and overlapping features with renal cell carcinoma (RCC). A 72-year-old man with a history of nephrolithiasis and no prior urologic surgery was evaluated for a right renal mass. Fine-needle aspiration yielded cloudy red fluid with scant epithelial cells and a fibromyxoid background. Immunohistochemistry showed positivity for PAX8, AMACR, CK7, and CA-IX, leading to a pre-operative diagnosis of RCC. Partial nephrectomy revealed a unilocular cyst with a solid component. Histology demonstrated fibromyxoid stroma with embedded spindled cells and focal tubular NA features; therefore, a final diagnosis of fibromyxoid nephrogenic adenoma was rendered. Fibromyxoid nephrogenic adenoma is a benign entity that can closely mimic RCC on cytologic and immunohistochemical grounds. Our findings underscore the diagnostic pitfalls in small-volume cytologic samples. Awareness of the cytologic features of FMNA and cautious interpretation of immunostains are essential to avoid misdiagnosis and unnecessary treatment.