Metastatic carcinoma in follicular adenoma of the thyroid gland

We report here two autopsy cases of carcinoma that metastasized to a thyroid follicular adenoma. The first was an 82-year-old man with sigmoid colon adenocarcinoma. Three years after a colectomy he died of multiple metastases. Autopsy revealed that the metastatic colonic carcinoma was located in a thyroid follicular adenoma. The second case was a 51-year-old man with primary lung adenocarcinoma. At the time of admission, he was inoperable because of multiple organ metastasis. He died 14 months after admission due to the carcinoma. Autopsy revealed a metastatic lung adenocarcinoma also located in a follicular adenoma, as in the first case. The phenomenon of tumor-to-tumor metastasis is rare. The mechanism is still unknown, and to our knowledge, only five cases of carcinoma metastatic to thyroid follicular adenoma have been documented in the literature.