Coexisting Medullary and Papillary Thyroid Carcinomas: A Case of Dual Neoplasia With a High Risk of Misdiagnosis

The simultaneous occurrence of more than two types of neoplasms is rare due to their significant phenotypic differences. Thyroid carcinoma is regularly associated with genetic alterations and endocrine syndromes. However, the etiology of the forms of papillary thyroid carcinoma (PTC)/medullary thyroid carcinoma (MTC) is still not fully understood. We present the case of a 60-year-old male with no significant history of thyroid disease who presented with dysphonia. Left vocal cord paralysis and a cervical tumor lesion were observed. A mixed medullary/papillary carcinoma was diagnosed. Surgical resection of the mediastinal tumor and hemithyroidectomy revealed a poorly differentiated blue cell MTC, with a tumor size of 6.5 x 6.4 x 4.4 cm and a Ki-67 proliferation index of 10%. The tumor was positive for cytokeratin AE1/AE3, carcinoembryonic antigen, synaptophysin, chromogranin, thyroid transcription factor 1 (TTF-1), S-100, and calcitonin. Metastasis was identified in a lymph node at the mediastinum, supporting a diagnosis of usual-type papillary thyroid carcinoma, with positivity for cytokeratin AE1/AE3, thyroglobulin, and TTF-1. Genetic tests related to hereditary cancer (Gencell Pharma, Bogotá, Columbia) were negative. The simultaneous presence of MTC and PTC in a patient is a rare event. The clinical characteristics and biological behavior of these cancer types can vary. The prognosis is directly related to the stage at presentation and the condition at the time of diagnosis.