Undifferentiated (Anaplastic) Thyroid Carcinoma: Exploring the Coexistence of Differentiated and Dedifferentiated Anaplastic Elements—Critical Insights from a Case Series and Review of Literature

Abstract Undifferentiated (anaplastic) thyroid carcinoma (ATC) is a rare thyroid malignancy, constituting <2% of all thyroid carcinomas. It is an aggressive tumor, manifesting as a rapidly growing neck mass with compressive symptoms. Given that ATC often arises from dedifferentiation of well-differentiated thyroid carcinoma (WDTC), it is also possible that WDTC and ATC elements coexist, necessitating a clear distinction between WDTC with anaplastic transformation and pure ATC, due to significant differences in prognosis and treatment. Emphasizing the significance of cytological evaluation, particularly with the assistance of fine-needle aspiration, we present three cases that underscore the importance of timely detection for achieving precise diagnoses and guiding subsequent management decisions.