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

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. 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. 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. 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.