SUN-597 A Case of Metastatic Medullary Thyroid Carcinoma Masquerading as a Pituitary Adenoma

Abstract

Introduction: Pituitary metastasis from systemic cancers are rare and can be challenging to manage. Breast and lung cancers are the most common primary tumors metastasizing to the pituitary. Medullary thyroid cancer (MTC) metastasis to the pituitary is exceedingly rare, with only a few cases described in the literature. We present a case of metastatic MTC that presented as pituitary mass and was later confirmed as metastatic MTC. Case: A 20-year-old Caucasian male presented with symptoms of fatigue, diarrhea and failure to thrive. He was found to have panhypopituitarism, and subsequent pituitary MRI showed a 1.7x1.8x2.5 cm pituitary mass. He underwent resection, and initial pathology revealed a pituitary adenoma (positive for synaptophysin , CAM5.2 , Ki-67 5%). Post-operatively, his hospital course was complicated by diabetes insipidus and sinus tachycardia, which prompted imaging of the chest. A chest CT scan demonstrated mediastinal lymphadenopathy, pulmonary nodules, and sclerotic bone lesions. Biopsy of the mediastinal mass revealed medullary thyroid carcinoma (pathology positive for AE1/AE3, CK7, synaptophysin, calcitonin, and TTF-1). He had elevated calcitonin (11,244 pg/ml n: 0-7.5 pg/ml) and CEA (1,103 ng/ml n: <5.0 ng/ml). A thyroid ultrasound showed a 6.3x1.5x1.3 cm thyroid nodule on the left with multiple suspicious lymph nodes. Given these new findings, pituitary tissue was stained with calcitonin and found to be positive. RET-proto-oncogene mutation M918T was present. Family history is unknown because the patient is adopted. Thyroidectomy has been deferred due to extensive tumor burden. He is currently participating in the phase two trial, “LIBRETTO-001" for the RET inhibitor Loxo-292, with the goal of decreasing tumor burden. Discussion: Diagnosing metastatic MTC involving the pituitary gland is particularly challenging when the primary tumor is not yet diagnosed. While usually clinically silent, certain symptoms raise the possibility of metastases to the pituitary. Diabetes insipidus, visual field deficits, and ophthalmoplegia have been shown to be presenting features. Diarrhea is also a common symptom of metastatic MTC. Primary pituitary adenomas can show overlapping morphology with MTC but are usually negative for TTF-1, calcitonin, and CEA on immunohistochemical analysis. Surgery is the mainstay of treatment but only curative in early stage disease. Adjuvant radiation therapy can be used for palliation and local control of advanced disease, while chemotherapy has been shown to provide limited benefit. Tyrosine kinase inhibitors can improve progression free survival in advanced disease. Specific patients, such as ours, can be referred to clinical trials for therapies that target the molecular pathogenesis of MTC. In summary, we present a case of metastatic MTC with an unusual presentation, intricate diagnosis, now undergoing experimental treatment.