Cytologic diagnosis of thyroid-like follicular carcinoma of the kidney: a case report.

Thyroid-like follicular carcinoma of the kidney (TLFCK) is a recently described subtype of renal-cell carcinoma that is not currently included in the World Health Organization classification. Few sporadic case reports and one series have been reported with emphasis on histopathologic features. However, cytological features have not been described in the literature to date. A 34-year-old male presented with a renal mass. An intraoperative smear preparation of the tumor revealed a hypercellular smear with cells arranged in sheets without any follicular, papillary, or acinar arrangement. The most striking feature was the presence of acellular eosinophilic material associated with the neoplastic epithelial cells in the background of the smear. Individual tumor cells were oval, round, and plasmacytoid with mild nuclear pleomorphism, finely stippled nuclear chromatin, and inconspicuous nucleoli with moderate amount of eosinophilic cytoplasm and rare nuclear grooves. It was unclear at the time of the intraoperative assessment of the smear if the acellular eosinophilic material represented metachromatic matrix-like extracellular material, mucin, colloid, amyloid, or hyaline material. The differential diagnoses included a primary renal-cell carcinoma versus a metastatic tumor. Subsequent histopathologic examination was diagnostic of a rare, recently described primary neoplasm of the kidney called TLFCK. This work is a retrospective evaluation of the cytological features of TLFCK. It is important for cytopathologists to be aware of this entity and its cytological features to render a correct diagnosis for adequate management of these patients.