# Thyroid-like Follicular Carcinoma of the Kidney: The Follicles Are out There in the Kidney—Now What?

## TL;DR

This paper presents a rare case of thyroid-like follicular carcinoma of the kidney and discusses its diagnostic challenges and clinical behavior.

## Contribution

The paper contributes a new case of TLFC-K and highlights its distinct immunohistochemical profile to aid in diagnosis.

## Key findings

- TLFC-K was diagnosed based on histological and immunohistochemical features excluding thyroid origin.
- The patient showed no signs of disease relapse at 5 months post-surgery.
- Folliculo-tubular patterns in kidney lesions can mimic thyroid metastases, complicating diagnosis.

## Abstract

Thyroid-like follicular carcinoma of the kidney (TLFC-K) is a rare primary kidney carcinoma with fewer than 60 reported cases. Current data suggest that TLFC-K has low malignant potential, with only a few reported cases of unfavorable clinical behavior. Histologically, TLFC-K is indistinguishable from kidney metastasis of well-differentiated follicular cell-derived thyroid carcinomas. Furthermore, folliculo-tubular patterns can be seen in different types of kidney lesions, making assessing follicular architecture in the kidney diagnostically challenging. We present a case of TLFC-K with a list of differential diagnoses. A hyperechoic tumor was found incidentally in the upper pole of the right kidney of a 66-year-old man. The patient underwent a radical nephrectomy. Histologically, the tumor was well-circumscribed, composed of follicular/tubular structures of different sizes filled with colloid-like material. Immunohistochemically, the absence of a positive reaction for thyroglobulin and TTF-1 excluded the secondary origin of the tumor from the thyroid. Tumor cells also showed diffuse positivity for vimentin and PAX8 and focal positivity for CK7 and CD10. The results of all other applied immunostaining tests did not align with those of different types of kidney tumors that may exhibit predominantly follicular patterns. Accordingly, TLFC-K was diagnosed. The patient shows no signs of disease relapse at the 5-month follow-up.