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.
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.
Reference:
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)