Anaplastic Thyroid Carcinoma and High-Grade Tall Cell Papillary Thyroid Carcinoma: Case Report of a Rare Association

Background and Clinical Significance: Anaplastic thyroid carcinoma (ATC) is a highly aggressive malignancy that may arise through dedifferentiation from pre-existing differentiated thyroid carcinomas. The tall cell subtype of papillary thyroid carcinoma (TC-PTC) represents an aggressive variant that has been involved in this tumor progression pathway. Case Presentation: We report on a rare case of ATC developed in association with a high-grade TC-PTC. A 67-year-old man presented with an enlarging anterior cervical mass. Imaging identified a suspicious nodule in the right thyroid lobe, and total thyroidectomy was performed. Histologic examination revealed a biphasic tumor composed of a nodular TC-PTC with high-grade features, contiguous with an infiltrative anaplastic carcinoma component. The anaplastic component showed marked pleomorphism, loss of thyroid differentiation markers, and an increased ki67 proliferation index. Multinucleated giant cells exhibited aberrant CD68 expression, without proliferative activity. Conclusions: This case illustrates the morphologic association between tall cell papillary thyroid carcinoma with high-grade features and anaplastic thyroid carcinoma, emphasizing diagnostic considerations within the framework of the WHO 5th edition classification.