A challenging case of enteric-type lung adenocarcinoma metastatic to the thyroid harboring RET-fusion diagnosed on fine-needle aspiration.

Enteric-type lung adenocarcinoma is a histological entity where the enteric component exceeds 50% and must show the expression of at least one immunohistochemical marker of enteric differentiation. The most common differential diagnosis is colorectal carcinoma. Molecular alterations less commonly observed in colorectal adenocarcinoma but more frequently associated with enteric-type lung adenocarcinoma can be helpful in differential diagnosis. In this report, we show a case of a thyroid lesion diagnosed on FNA as a metastasis from colorectal carcinoma and then recognized as a metastasis from enteric-type lung adenocarcinoma thanks to the identification of a RET-fusion through NGS testing. This is the first case of enteric-type lung adenocarcinoma with RET-fusion reported.