Thyroid-like follicular carcinoma of the kidney with metastases to the lungs and retroperitoneal lymph nodes.

Thyroid-like follicular carcinoma of the kidney is an extremely rare variant of renal cell carcinoma. Most previously reported cases presented as incidental small tumors confined to the kidney. Here we report a unique case in which the patient presented with flank pain and hematuria. Imaging studies demonstrated a large tumor in the right kidney and metastases to the lungs and retroperitoneal lymph nodes. Both the renal tumor and the sampled lung metastasis were composed almost entirely of follicles with dense, colloid-like material resembling thyroid follicular carcinoma. However, no lesion was found in the thyroid gland; and the patient's thyroid function test results were normal. The tumor cells were immunoreactive for PAX2 and PAX8 but lacked reactivity for thyroglobulin and thyroid transcription factor-1. To our knowledge, this is the first case of thyroid-like follicular carcinoma of the kidney to be initially associated with marked symptoms and widespread metastases, providing evidence that this rare variant of renal cell carcinoma can be clinically aggressive.