SAT-585 Presence of Metastatic Medullary Thyroid Cancer without a Distinct Thyroid Nodule: A Diagnostic Challenge

Introduction Neuroendocrine tumors originate from the GI tract, lungs or endocrine glands. Diagnosis can be done by different imaging modalities including Octreoscan, MIBG scan, FDG PET/CT and Ga-Dotatate PET/CT. Case 52 year old female with past history of hypertension, presented to an outside facility with complaint of a right lower neck mass, hoarseness and cough. Thyroid ultrasound identified a 0.3 cm right lobe calcific density with no discrete nodules. Neck CT showed right thyroid calcification and a 2.5 cm right supraclavicular mass. A core needle biopsy of the mass was consistent with metastatic low grade neuroendocrine carcinoma. Immunostains were positive for EP-CAM, TTF-1, synaptophysin, CD56 and CK7. Metastatic pulmonary carcinoma was suspected. She was seen at our institution for a multidisciplinary team evaluation. Pathology slides for additional stains were unavailable. Whole body CT showed low suspicion mediastinal lymph nodes. FDG PET/CT showed a 2.6 cm right neck hypermetabolic mass and a tiny calcified density in the right thyroid lobe, but no metastatic disease elsewhere. Endoscopic ultrasound-guided transbronchial biopsy of the mediastinal nodes was negative for cancer. An Octreotide scan showed only right neck mass hypermetabolism. Gallium-68 Dotatate scan had just become available and was able to identify increased uptake in the right supraclavicular region and an additional focus of uptake within the superior aspect of the right thyroid lobe. Fine-needle aspiration of the right thyroid focus was consistent with medullary thyroid carcinoma. Calcitonin was 1268 pg/ml, CEA 1.7 ng/ml and RET proto-oncogene was positive (p.C609Y). She had normal parathyroid hormone, calcium and plasma metanephrines. Total thyroidectomy was performed. Pathology was consistent with right neck metastatic medullary thyroid carcinoma with 18/27 lymph nodes involved. Multifocal medullary thyroid carcinoma from 0.2-0.6 cm was found in the right lobe. The left lobe had a 0.2 cm focus of medullary thyroid carcinoma with 1/2 lymph nodes involved. Her most recent neck ultrasound showed a persistent, stable hypoechoic 0.5 cm right lateral neck lesion. Most recent neck CT was unremarkable. PET/CT done 2 months ago showed only mildly prominent mediastinal lymph nodes that are stable compared with 2 years ago. Calcitonin decreased initially from 1268 pg/ml to 321 pg/ml and has now plateaued in the low 500s. CEA and other biomarkers are normal. Patient's daughter and son tested positive for the RET proto-oncogene mutation. Conclusion Metastatic medullary thyroid carcinoma without a discrete thyroid nodule is an uncommon presentation. Our patient presented with a metastatic neuroendocrine neck mass and conventional imaging modalities failed to identify the primary site, leading to delay in proper treatment. Ga-Dotatate PET/CT may improve management of patients with neuroendocrine tumors.