Abstract
Background: To report a rare case of mixed medullary thyroid cancer and follicular cancer.
Case Report: Sixty year old male was referred for an incidental thyroid nodule on carotid ultrasound, confirmed to be 1.1 cm mixed cyst on CT scan and a cold nodule on thyroid scan and 1.7 cm complex thyroid mass with intra-nodular vascularity, central calcification and an irregular posterior border on thyroid US. No evidence of thyroid dysfunction, compressive symptoms or radiation exposure. FNA of the nodule showed a follicular lesion. Calcitonin and CEA were elevated at 584 pg/L and 15.0 ng/mL respectively. Total thyroidectomy with central neck dissection done revealed a 1.7 cm medullary thyroid cancer with microscopic invasion into the surrounding thyroid tissue. Ipsilateral level VI node was positive for medullary thyroid cancer. Four weeks post-op Ultra sound neck and thyroid bed showed two right-sided lymph nodes less than 1 cm, both adjacent to the carotid arteries and US guided FNA was attempted with insufficient material. Given the soft-tissue invasion and the persistence of lymph nodes the patient had extensive bilateral neck dissection. One of forty lymph nodes at ipsilateral node was positive with an area of medullary thyroid cancer with an adjacent area of follicular thyroid epithelial differentiation.
Conclusions: This is the first reported case in the English literature of an area of follicular thyroid differentiation in a lymph node adjacent to a metastatic focus of medullary thyroid cancer in a patient without an identified primary follicular thyroid cancer.

Keywords: Medullary Thyroid Cancer, Follicular thyroid cancer, mixed thyroid cancer