Abstract #1402263: A Malignant Duo: Mixed Medullary and Follicular Variant Papillary Thyroid Cancer

Abstract

Medullary thyroid cancer (MTC) is rare and only constitutes 5-10% of all thyroid malignancies. Mixed MTC and papillary thyroid cancer (PTC) are even more rare and are only found in less than 1% of MTC cases. These malignancies arise from different cell origins, follicular cells of endoderm and parafollicular C cell of neural crest. It is not clear if the coexistence of these tumors in the same patient is a coincidence or if there is a common genetic culprit. We present a case of a lady with metastatic mixed MTC and PTC. A 61-year-old woman with history of Hashimoto disease was found to have bilateral thyroid nodules, the largest (1.7cm) was in the right lobe. This nodule met fine needle aspiration (FNA) biopsy criteria and was found to have a follicular neoplasm of undetermined significance. The patient elected to pursue total thyroidectomy instead of lobectomy given presence of bilateral nodules. Postoperative pathology showed mixed medullary carcinoma (pT3b) and follicular variant papillary thyroid microcarcinoma (pT1a) involving the right lobe with positive anterior and posterior margins and lymphovascular invasion. Preoperative calcitonin was not checked. However, post-thyroidectomy calcitonin was 1599 pg/mL. She underwent central and right lateral neck dissection which showed 27 out of 35 lymph nodes were positive for malignancy. Postoperative calcitonin dropped to 38.7 pg/mL. She then established care in our endocrine clinic. Screening for pheochromocytoma and primary hyperparathyroidism was normal. She underwent external beam radiation of the neck. A year after her initial surgery, her neck ultrasound and computed tomography (CT) studies show no signs of local or distant anatomic recurrence. Her thyroglobulin level remains undetectable, CEA within normal range, and calcitonin stable at about 20 pg/mL. She is on levothyroxine 100mcg daily with TSH at suppression goal of < 0.1 mIU/L. Mixed PTC and MTC is poorly studied due to its rarity. The origin of these mixed tumors is unclear, but some suggest that they arise from neoplastic changes of remnant multipotent cells in the thyroid. While patients with PTC often have a favorable prognosis following surgical therapy, MTC has a more aggressive course. A retrospective study of 183 patients suggests that this entity is more common in patients older than 45 years and is associated with a better survival than MTC alone. We suggest monitoring patients like ours for both MTC and PTC, as if present in isolation. Our case will highlight the clinical aspects of this condition and our current knowledge of its pathophysiology.