PDTC are malignant tumors that show limited follicular cell differentiation and are intermediate in morphology and biological behavior between DTC and ATC. PDTC can be associated with varying proportions of differentiated carcinoma components, but the presence of even 10% of PDTC components has been shown to be associated with aggressive behavior and a poor prognosis. The Ki-67 index of PDTC is usually in the range of 10% to 30% and is usually positive for BCL2, CyclinD1, and focally positive for P53, P21 and P27. The differential diagnosis mainly includes MTC, parathyroid carcinoma and carcinoma metastatic to the thyroid gland.
ATC is a highly aggressive malignancy composed of undifferentiated thyroid follicular cells. It typically presents as a rapidly enlarging, hard, fixed neck mass with extensive invasion of surrounding tissues, and approximately 30% to 40% of patients have distant metastases such as lung, bone and brain. The main histologic patterns are sarcomatoid, tumorigenic, and epithelioid, which may occur singly or in different proportions, or with focal squamous or heterologous differentiation; they are usually accompanied by necrosis, numerous nuclear schwannomas, and vascular invasion. Immunohistochemistry: TTF1 and TG are usually negative, PAX8 is positive in approximately half of cases, CK can be positive in areas of epithelioid differentiation, and LCA, myogenic markers, and melanoma markers are mainly used for exclusionary diagnosis. Differential diagnosis: other types of highly malignant tumors such as myogenic sarcoma, malignant melanoma, and large cell lymphoma. Highly malignant primary thyroid tumors of non-follicular and parafollicular cell origin are also generally classified as ATC, such as squamous cell carcinoma, sarcoma, mucinous epidermoid carcinoma, etc.