Endocrine Abstracts

JOINT3972

Introduction: Thyroid carcinomas represent the most common endocrine malignancies, with papillary thyroid carcinoma (PTC) accounting for the majority of cases. Medullary thyroid carcinoma (MTC), arising from parafollicular C cells, is less frequent and follows a distinct clinical course. The coexistence of MTC and PTC in the same patient is an unusual finding with significant implications for diagnosis, genetic screening, and treatment. Understanding the interaction between these malignancies is crucial for optimizing outcomes.

Case Presentation: A 70-year-old male, ex-smoker with a history of arterial hypertension, and coronary artery disease was under investigation for a stable pulmonary nodule. During the work-up a PET DOTA-NOC scan revealed intense radiotracer uptake in the superior pole of the right thyroid lobe, with no uptake in the pulmonary nodule, prompting referral to endocrinology for further evaluation. Biochemical evaluation revealed normal TSH, high calcitonin (285pg/ml; N: <9.52) and CEA (5.3ng/ml; N:0-3) levels. Urinary metanephrines were normal. Cervical ultrasound showed two EU-TIRADS 4 thyroid nodules, on the right with 15x8x14mm and on the left with 10x8x11mm. FNA of the right nodule was consistent with medullary thyroid carcinoma. In this context, the patient underwent total thyroidectomy with central compartment neck dissection. Histopathology disclosed a 9mm MTC confined to the thyroid (pT1aN0R0) and two out of seven central lymph nodes with PTC metastasis, the largest with 3mm and extranodal extension. After full inclusion of the surgical specimen an incidental 3mm PTC was identified in the left lobe (pT1aN1aR0, ATA high risk). Postoperatively, calcitonin levels declined to 1.6 pg/ml, indicating a favorable response. The patient is currently awaiting genetic testing and is scheduled to undergo radioiodine therapy with 150mCi for the PTC.

Discussion: The synchronous occurrence of medullary and papillary thyroid carcinomas is rare, accounting for approximately 19% of all MTC cases and 0.28% of all PTC cases. While MTC originates from parafollicular C cells and is frequently associated with RET mutations, PTC arises from follicular cells and commonly harbors BRAF or RAS mutations. The coexistence of these two malignancies raises questions about potential shared pathogenic mechanisms. Classic genetic alterations found in isolated MTC and PTC do not seem to play a role in synchronous cases, suggesting independent oncogenic pathways. Although the simultaneous presence of these malignancies does not appear to significantly alter their individual clinical behaviors, accurate diagnosis remains challenging. This underscores the need for thorough biochemical and histopathological evaluation to ensure proper management and follow-up.