SUN-323 Metastasis to the Clivus - a Unique Presentation of Metastatic Medullary Thyroid Cancer Associated with Multiple Endocrine Neoplasia Type 2A

Abstract Disclosure: D. Al-Souri: None. E. Chuki: None. P. Chittiboina: None. D. Smart: None. J. Diamond: None. M. Laws: None. P. Veeraraghavan: None. S. Gubbi: None. J. Klubo-Gwiezdzinska: None. Introduction: Medullary thyroid carcinoma (MTC) is a rare neuroendocrine tumor arising from parafollicular C cells of the thyroid that can be associated with Multiple Endocrine Neoplasia type 2 (MEN2 A and MEN2B), driven by germline RET proto-oncogene pathogenic variants. Managing advanced metastatic MTC in MEN2A patients poses clinical challenges due to its aggressive nature and propensity for widespread metastasis, especially in patients with high-risk RET pathogenic variants. We present one such patient with an unusual metastatic site, emphasizing the diagnostic, therapeutic, and follow-up strategies. Clinical Case: A 49-year-old female with MEN2A (germline RET C634Y pathogenic variant with 4 family members affected by MEN2A) and a history of MTC was referred for evaluation of disease progression. In 2010, she underwent a total thyroidectomy for a 3.8 cm MTC, without modified neck dissection, given the presence of widespread metastatic disease to the liver, bones, and persistent neck lymphadenopathy. In 2013, the patient also had right total adrenalectomy and partial left adrenalectomy for bilateral pheochromocytomas. Subsequently, based on a failed ACTH stimulation test, she was diagnosed with adrenal insufficiency after complaints of persistently low blood pressure and low energy. The patient also reported intermittent dizziness and headaches but without neurological deficits Comprehensive evaluation including biochemical and imaging studies (CT neck/chest/abdomen/pelvis, FDG PET, MRI brain) in 2024 revealed new metastasis to the clivus. Calcitonin levels were markedly elevated (22,931 pg/mL). Given a unique presentation with the expansile clivus lesion, a sublabial transsphenoidal resection was performed, and postoperative imaging revealed that near total resection was achieved. Postoperatively, the patient developed transient cranial nerve VI paresis, managed with high-dose corticosteroids, followed by physiologic hydrocortisone replacement. Subsequent management includes radiotherapy planned for January 2025 and therapy with RET inhibitor. Conclusions: This case highlights the aggressive behavior of a subset of MTCs, with clivus involvement being an unusual metastatic site. Clival metastases are extremely rare, representing only 0.04% of all intracranial tumors and most commonly originating from prostate cancer and gastrointestinal malignancies. There are only two reported cases of papillary thyroid cancer with clival metastases, while our patient represents the first reported metastatic disease to the clivus from MTC. Managing MEN2A with metastatic disease and bilateral pheochromocytomas presents unique challenges requiring a multidisciplinary approach. Surgical resection remains pivotal, but adjunctive therapies should be considered based on disease progression. Presentation: Sunday, July 13, 2025