SUN-338 Double Impact: Convergent Thyroid Carcinomas

Abstract Disclosure: J. Tamayo Acosta: None. G. Rivera Rodriguez: None. R. Trinidad-Hernandez: None. M. Acosta-Guillot: None. Collision tumors of the thyroid, defined by the coexistence of two distinct neoplastic entities within a single gland, represent a rare and diagnostically formidable challenge with profound implications for clinical management. We report the case of a 60-year-old male with controlled hypertension who was incidentally found to have a palpable neck mass. On physical examination, a firm, non-tender, irregular mass that moved with deglutition was identified in the right thyroid region. Despite normal thyroid function tests, high-resolution ultrasonography revealed multiple nodules in the right lobe, including a large heterogeneous solid nodule with increased vascularity, calcifications, and an incomplete hypoechoic rim. Ultrasound-guided fine-needle aspiration (FNA) of the dominant nodule demonstrated cytologic features diagnostic for malignant papillary thyroid carcinoma (PTC).The patient subsequently underwent total thyroidectomy. Histopathological evaluation of the right thyroid lobe unexpectedly disclosed an encapsulated papillary carcinoma of the follicular variant (2.4 cm) with capsular invasion, concurrently with a 1.3 cm focus of medullary thyroid carcinoma (MTC). Immunohistochemical analysis provided definitive differentiation: the papillary component exhibited robust expression of thyroglobulin and thyroid transcription factor-1 (TTF-1), whereas the medullary component was positive for carcinoembryonic antigen (CEA) and calcitonin.This case underscores several critical teaching points. First, collision tumors should be strongly considered when clinical findings, such as a palpable, irregular thyroid mass, are discordant with normal laboratory evaluations. Second, although ultrasonography and FNA remain indispensable for the initial assessment of thyroid nodules, definitive diagnosis necessitates comprehensive histopathological and immunohistochemical analysis. Third, optimal management of these complex thyroid malignancies requires a multidisciplinary approach—including surgical oncology, endocrinology, and genetic counseling—to tailor therapeutic strategies that address the unique pathology of each tumor component. Our report not only enriches the limited literature on thyroid collision tumors, particularly those involving concurrent papillary and medullary variants, but also emphasizes the need for heightened clinical vigilance and individualized treatment protocols to improve patient outcomes in this challenging subset of endocrine malignancies. Presentation: Sunday, July 13, 2025