Multifocal Medullary and Papillary Thyroid Carcinoma Occurring as a Collision Tumor: A Case Report.

Collision tumors are rare tumors comprising two morphologically distinct tumors within the same organ without histological admixture. Thyroid collision tumors are extremely rare. We present a case of a 64-year-old male patient with a radiologically suspicious, TI-RADS-TR4 lesion in the right lobe of the thyroid. Fine needle aspiration cytology (FNAC) from the lesion was diagnosed as medullary thyroid carcinoma. Total thyroidectomy with central and right lateral neck dissection was performed. On histopathological evaluation, a collision tumor was identified. Components of the collision tumor were multifocal medullary thyroid carcinoma (MTC) and multifocal infiltrative follicular variant of papillary thyroid carcinoma (PTC). Several hypotheses have been suggested regarding the pathogenesis of the collision tumor. Further management and prognosis of the tumor depend on the component with the higher stage and more aggressive behavior. The case report emphasizes the need for thorough sampling of uninvolved areas in the specimen for microscopic evaluation and staging of each component.