Primary thyroid-like follicular carcinoma of the kidney: report of 6 cases of a histologically distinctive adult renal epithelial neoplasm.

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.