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Case of the Month: February 2022

posted: February 14, 2022

This is a 64 year old female with a 3.9 cm left renal mass incidentally discovered on CT scan during workup for microhematuria. The mass was cystic and within interpolar region of the left kidney. A partial nephrectomy was performed.

February 2022 Case Pic
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Pathologic Description

Grossly, a well-circumscribed and cystic 3.9 cm mass filled with yellow viscous material was seen. On microscopic examination, the tumor is composed of micro- and macro-follicles with abundant eosinophilic colloid-like material. The lining cells are cuboidal with moderate amphophilic to eosinophilic cytoplasm, round to ovoid nuclei, and fine chromatin. Nucleoli are inconspicuous with a few cells showing prominent nucleoli. The tumor is organ confined. No nuclear pleomorphism, mitosis, or renal sinus/perinephric fat invasion is seen. Immunohistochemical stains show tumor cells are positive for PAX8, CK7 and CAM5.2; while negative for TTF-1, thyroglobulin, CD10 and AMACR.

Final Diagnosis: Primary thyroid-like follicular carcinoma of the kidney
Primary thyroid-like follicular renal cell carcinoma is a rare entity that is histologically similar to primary thyroid follicular carcinoma but lacking thyroid immunohistochemical stains. Most cases show an indolent course; however, a few cases of distant metastasis and perinephric fat invasion have been reported. There is a slight female predisposition with wide age range (19-85 years). Main differentials include metastatic follicular carcinoma from a thyroid primary and metastatic follicular carcinoma arising in struma ovarii in females. Immunohistochemical staining with TTF-1 and thyroglobulin (negative in primary thyroid-like carcinoma of the kidney) is used to distinguish between these entities. PAX8 immunohistochemical stain is positive in both primary thyroid-like follicular carcinoma of the kidney and metastatic thyroid follicular carcinoma. Due to its indolent course, surgical resection is the treatment of choice.
Amin MB, Gupta R, Ondrej H, McKenney JK, Michal M, Young AN, Paner GP, Junker K, Epstein JI. Primary thyroid-like follicular carcinoma of the kidney: report of 6 cases of a histologically distinctive adult renal epithelial neoplasm. Am J Surg Pathol. 2009 Mar;33(3):393-400. doi: 10.1097/PAS.0b013e31818cb8f5. PMID: 19047894.

Case contributed by Dr. Ruchi Jha and Dr. Faisal Saeed (Emory University Hospital, Department of Pathology)