Metastatic mesothelioma to the thyroid

A 69 year-old male patient with a history of malignant mesothelioma treated with chemotherapy and surgical resection with removal of the right lung and right pleural pneumonectomy was clinically in remission for 1 ½ years. A positron emission tomography (PET) scan revealed limited uptake in the right pleural space, thought to represent post-surgical changes, and uptake in the left thyroid lobe. Thyroid ultrasound revealed a solid left lobe nodule with peripheral vascularity and absent microcalcifications. Fine needle aspiration cytology showed a microfollicular arrangement of cytologically bland cells with variable Hürthle cell changes initially interpreted as suspicious for Hürthle cell neoplasm. Review at multidisciplinary conference raised the possibility of metastatic mesothelioma, supported by immunohistochemical studies in the cell block. The patient opted for left hemithyroidectomy with isthmusectomy which confirmed malignant mesothelioma. Repeat PET scan 6 months later revealed no further uptake in the thyroid bed, with limited uptake in the right pleural space. Metastatic tumors to the thyroid are uncommon with only one previous description of metastasis to the thyroid by mesothelioma. Metastasis of cytologically low grade tumors such as mesothelioma present problems for cytology due to the potential for overlap with the variable appearances of thyroid neoplasms. The value (if any) of ancillary tests, including mutation testing, expression profiling and immunohistochemistry is discussed.