Analysis of clinical and histopathological features in concurrent papillary and medullary thyroid carcinoma.

OBJECTIVE: Concurrent papillary thyroid carcinoma (PTC) and medullary thyroid carcinoma (MTC) is a rare entity. We aimed to investigate the clinicopathological features, treatment, and prognosis of this coexistence. METHODS: We retrospectively studied 23 patients with concurrent PTC and MTC presenting as collision tumors (referred to as PTC/MTC) between 2011 and 2023 at our center. RESULTS: Among 197 MTC patients, 23 (11.7%) had PTC/MTC. Preoperative assessments revealed an 88.9% positivity rate for calcitonin and a 54.5% positivity rate for carcinoembryonic antigen. Thyroid ultrasonography showed more than one nodule suspicious of malignancy in 73.9% of the patients and detected nodal metastases in 63.9%. The MTC component had a higher frequency of abundant blood-flow signals than the PTC component (p = 0.009). Fine-needle aspiration cytology (FNAC) diagnosed only one tumor component in 75.0% cases. Among the 23 cases with PTC/MTC, 15 (65.2%) were identified as medullary thyroid microcarcinoma (micro-MTC), 19 cases (82.6%) as papillary thyroid microcarcinoma (micro-PTC). The proportion of micro-MTC in collision tumors was significantly higher than that in all MTC cases (p < 0.001). Further comparison between micro-MTCs in PTC/MTC and those not associated with PTC demonstrated no significant differences in clinicopathological features and prognosis. CONCLUSIONS: In this cohort, PTC/MTC was found in approximately 1 in 10 MTC patients. Multiple suspicious malignant thyroid nodules with abundant blood-flow signals warrant vigilance for concurrent PTC and MTC, even if FNAC detects only one tumor component. Micro-MTC accounts for a high proportion of PTC/MTC cases, and its clinicopathological features are not influenced by the presence of a PTC component.