Thyroid follicular carcinoma-like renal tumor: a case report with morphologic, immunophenotypic, cytogenetic, and scintigraphic studies.

In this report, a rare renal tumor that morphologically resembles a thyroid follicular carcinoma is described. To date, this subtype has not been integrated into a known form of renal carcinoma. A 29-year-old female patient without relevant family or social history underwent nephrectomy because of a renal tumor measuring 5 cm by the largest diameter. The macroscopically white-yellow tumor showed follicular structures with abundant eosinophilic colloidal material and focal papillary differentiation by light microscopy. Immunohistochemically, the tumor cells stained positively for cytokeratin (CK-7, CK-20, CAM 5.2) and vimentin. CD-10, CD-117, thyroid transcription factor-1, and thyreoglobulin remained completely negative. Chromosomal losses of 1, 3, 7, 9p21, 12, 17, and X were detected by fluorescence in situ hybridization. Scintigraphs showed an inconspicuous thyroid gland and no extrathyroidal pathological accumulations, making metastatic spread to the kidney highly unlikely. To our knowledge, this is the second fully documented case of a thyroid follicular carcinoma-like renal tumor. This uncommon variant is important to keep in mind to prevent unnecessary or inappropriate treatment.