Original ArticlesPrimary Thyroid-like Follicular Carcinoma of the Kidney: Report of 6 Cases of a Histologically Distinctive Adult Renal Epithelial NeoplasmAmin, Mahul B. MD*; Gupta, Ruta MD*; Ondrej, Hes MD†; McKenney, Jesse K. MD‡; Michal, Michal MD†; Young, Andrew N. MD, PhD§; Paner, Gladell P. MD*; Junker, Kerstin MD∥; Epstein, Jonathan I. MD¶Author Information *Department of Pathology, Cedars-Sinai Medical Center, Los Angeles ‡Department of Pathology, Stanford University Medical Center, Stanford, CA †Department of Pathology, Charles University Hospital, Pilsen, Czech Republic §Department of Pathology and Laboratory Medicine, Emory University School of Medicine, Grady Health System, Atlanta, GA ∥Laboratory of Molecular Biology, Department of Urology, Friedrich Schiller University, Jena, Germany ¶Department of Pathology, The Johns Hopkins Hospital, Baltimore, MD Presented in part at the 93rd Annual Meeting of the United States and Canadian Academy of Pathology (USCAP) in Vancouver, British Columbia, Canada, on March 8, 2004 and 96th Annual Meeting of USCAP in San Diego, CA, on March 27, 2007. Correspondence: Mahul B. Amin, MD, Department of Pathology and Laboratory Medicine, Cedars-Sinai Medical Center, 8700 Beverly Blvd. Suite 8728, Los Angeles, CA 90048 (e-mail: firstname.lastname@example.org). The American Journal of Surgical Pathology: March 2009 - Volume 33 - Issue 3 - p 393-400 doi: 10.1097/PAS.0b013e31818cb8f5 Buy Metrics Abstract Thyroidization of kidney reminiscent of thyroid follicles with accumulation of inspissated colloid-like material in renal tubules is a hallmark of chronic pyelonephritis. We identified 6 tumors in the kidney, distinct from currently known subtypes of renal cell carcinoma, with a striking histology that closely mimicked well-differentiated thyroid follicular neoplasms and raised the possibility of metastatic follicular thyroid carcinoma. Three occurred in males and 3 in females with an age range of 29 to 83 years and size range from 1.9 to 4 cm. All tumors were encapsulated and exclusively demonstrated follicular architecture comprising of microfollicles and macrofollicles containing inspissated colloid-like material. A minor component of small tightly packed follicles devoid of secretions was also noted. The follicles were lined by cells with moderate amphophilic to eosinophilic cytoplasm with round nuclei and occasional prominent nucleoli. The tumors were nonimmunoreactive with thyroglobulin and thyroid transcription factor 1 and for markers contemporarily used for renal differentiation. The tumors had a gene expression profile distinct from clear cell and chromophobe renal cell carcinoma. Comparative genetic hybridization failed to reveal cytogenetic alterations. Mean follow-up of 47.3 months (range: 7 to 84 mo) showed that 5 patients had no evidence of disease and 1 developed a metastasis to the renal hilar lymph nodes in which the follicular architecture with colloid was retained. Thyroid-like follicular renal cell carcinoma represents a unique histologic subtype of renal cell carcinoma of low malignant potential and its primary importance is to distinguish it from metastatic carcinoma from the thyroid. © 2009 Lippincott Williams & Wilkins, Inc.