Background: Anaplastic thyroid carcinoma (ATC) is rare but is one of the most aggressive and lethal human malignancies. Cytologically, ATC has a variable morphologic appearance, including squamoid, giant, spindled and pleomorphic cells. The coexistence of ATC and differentiated or poorly differentiated thyroid carcinoma has been described and usually is diagnosed when the disease is locally advanced.
Case: We describe a case of surgically resectable, encapsulated, well-circumscribed ATC occurring in association with a better differentiated follicular carcinoma diagnosed by fine needle aspiration in a patient exposed to external ionizing radiation.
Conclusion: Encapsulated variants of anaplastic carcinoma can be seen in association with lower grade thyroid carcinoma such as follicular carcinoma. Accurate diagnosis is dependent on adequate sampling.