12396 A Case Of Medullary Thyroid Carcinoma Without Germline RET Mutation

Abstract Disclosure: I. Marranzini Rodriguez: None. M. Lemelman: None. L. Canham: None. Background: Medullary thyroid cancer (MTC) is a neuroendocrine tumor that originates from the parafollicular C cells of the thyroid gland. MTC accounts for approximately 5% of thyroid cancer in children, and the majority of MTC cases are associated with the autosomal dominant tumor predisposition syndrome Multiple Endocrine Neoplasia 2 (MEN2). Sporadic MTC is uncommon in the pediatric population and associated with somatic mutations of RET or RAS (1). Existing management options for metastatic disease have demonstrated some benefit, however, with systemic side effects. Selpercatinib, a highly selective RET kinase inhibitor, is FDA approved for patients >= 12 years of age. Here we present a case of a 10-year-old patient with metastatic MTC with encouraging results. Clinical Case: A previously healthy 10 year 2-month-old male presented with left-sided neck swelling. Imaging showed a 9.7 cm solid mass spanning nearly the entire left neck, occluding the left internal jugular vein, extending to the upper chest, and resulting in high-grade narrowing of the trachea near the thoracic inlet; the mass involved the left thyroid gland and engulfed the left common carotid. His initial lab evaluation showed an elevated TSH of 10.10 mcU/mL (0.6-5.5 mcU/mL) and Free T4 1.22 ng/dL (0.9-1.67 ng/dL). Tumor markers revealed a calcitonin of 32,264 pg/mL (normal range <= 6.0 pg/mL) and carcinoembryonic antigen (CEA) of 200 ng/ml (normal range 0 – 3.4 ng/mL). He underwent biopsy of the mass which showed medullary thyroid carcinoma (MTC) and genetic evaluation of tumoral tissue showed RET c.2753T>C, p.M918T somatic mutation. Evaluation for pheochromocytoma and hyperparathyroidism, given association of MTC with MEN2, was negative. Genetic testing was negative for germline RET mutations. He was started on selpercatinib, 120 mg every 12 hours. Follow up imaging after 2 months of therapy demonstrated a significant decrease in tumor burden. Approximately 5 months after starting he had a decrease in CEA to 14.8 ng/mL and calcitonin to 69 pg/mL, prior to surgery. Given the risk of locoregional recurrence he underwent total thyroidectomy with lateral neck dissection. However, the tumor was noted to be adherent to the trachea, encasing the left recurrent laryngeal nerve, and left carotid artery preventing full resection. He was started on levothyroxine at 62.5 mcg daily on POD#1 and re-started on selpercatinib approximately 2 weeks post-operatively. Conclusion: We present a case of RET-positive metastatic medullary thyroid carcinoma without germline RET mutation successfully treated with selpercatinib. Long-term follow-up for sustained response and tolerability is needed. Reference: 1. Bauer, Andrew J. “Pediatric Thyroid Cancer: Genetics, Therapeutics and Outcome.” Endocrinology and metabolism clinics of North America vol. 49,4 (2020): 589-611. doi:10.1016/j.ecl.2020.08.00 Presentation: 6/2/2024