Collision thyroid tumors: a case report of coexisting papillary and follicular carcinoma.

Collision tumors of the thyroid gland are defined by the coexistence of two or more histologically and morphologically distinct neoplasms, separated by normal thyroid tissue. This phenomenon is rare, with the most frequently reported combination being the coexistence of medullary carcinoma and papillary carcinoma. This case reports a 73-year-old female patient with an asymptomatic thyroid nodule. Ultrasonographic examination revealed a solid, isoechoic, well-defined nodule with a halo and peripheral vascularization, measuring 4.3 × 2.5 cm in the left lobe. Fine-needle aspiration cytology classified the nodule as Bethesda IV (follicular tumor). Following total thyroidectomy, histopathological analysis identified papillary carcinoma in the isthmus and minimally invasive follicular carcinoma in the left lobe.