SUN-596 A Rare Case of Metastatic Medullary Thyroid Carcinoma to Pituitary Gland

Background:&nbsp;Metastasis to the hypothalamus & pituitary gland account for 1-2% of sellar masses. Pituitary metastasis of medullary thyroid carcinoma (MTC) accounts for only 6 published cases in the literature. Imaging alone cannot diagnose metastasis & requires resection for histochemical identification. We describe a case of metastatic MTC presenting with 3(rd)&nbsp;nerve palsy & pituitary lesion. Case Presentation:&nbsp;63 y.o. man with 1-month history of bilateral retro-orbital headache, diplopia & left eye ptosis presented to the hospital for second evaluation. He had history of MTC (5 x 3 x 2.5 cm), h/o thyroidectomy 5 years prior & known metastatic non-progressive lung lesions on CT. Medications included levothyroxine & citalopram. He had no personal or family history of MEN syndromes. 2 weeks prior he went to another hospital for same complaints. Initial MRI of head found a parasellar 1.6 cm mass extending to sella. Neurosurgery did not recommend intervention at that time. At our hospital, dedicated pituitary MRI this time showed a primary intrasellar lesion displacing normal pituitary gland, without bony erosion, favoring a pituitary macroadenoma with bilateral cavernous sinus invasion. No optic optic chiasm involvement was noted. Calcitonin level was 2582 pg/mL (1580 pg/mL 10 months ago), CEA 17.0 ng/mL [<3.0 ng/mL]. Hormonal workup did not indicate functional adenoma - instead showed partial hypopituitarism, with LH 1.0 mIU/mL [1.24-8.62 mIU/mL], FSH 2.90 mIU/mL, am free testosterone 7.9 pg/mL [35 - 155 pg/mL], total testosterone 522 ng/dL [250-1100 ng/dL], TSH 0.059 [ 0.28-3.89 uIU/mL], free T4 0.72 ng/dL [0.58-1.64 ng/dL], IGF-1 230 [41-279 ng/mL], am cortisol of 8.4 mcg/dL [6.7-22.7 mcg/dL]. Ophthalmology consult revealed CN III palsy with left ptosis, without visual field deficits, indicating mass compression of CN III. Due to persistent headache, 2(nd) neurosurgical opinion was sought & transphenoidal resection performed. Initial frozen section suggested pituitary adenoma. Gross pathology demonstrated calcitonin, chromogranin A & synaptophysin immunoreactive cells consistent with metastatic MTC. Post operative MRI demonstrated only small residual right sellar & cavernous component. Daily local conventional fractionated radiation was started with Rad/Onc. Tyrosine kinase inhibitor therapy will be considered after post-radiation MRI & genetic testing for&nbsp;RET&nbsp;mutations. Discussion: Our patient’s clinical presentation of severe headache & rapid onset of diplopia was not consistent with pituitary adenoma, and the MRI result was a confounder. Conclusion:&nbsp;Although pituitary metastasis is rare, it should be considered in the differential in the appropriate clinical context. For MTC, active surveillance with imaging and biochemical markers is critical for management. Doubling time of calcitonin <1 year and rising CEA levels should prompt suspicion for disease progression.