Undifferentiated (anaplastic) thyroid carcinoma and its mimics: a report of 59 cases

Anaplastic thyroid carcinoma (ATC) is relatively rare. A correct pathologic diagnosis is mandatory to exclude less aggressive entities, and guide management. We describe our experience with the fine needle aspiration (FNA) cytomorphology of ATC and potential mimics.

Cytology files were searched to recover all cases of ATC/suspicious for ATC. Surgical pathology files were searched for ATC cases having corresponding cytology. FNA was performed using standard technique.

Of 59 FNA cases, 34 were correctly diagnosed as ATC (or possible ATC); 22 were false negative, and 3 false positive. All but 2 patients were confirmed by tissue biopsy or clinical course. Of 56 true ATC cases [51 patients (mean age: 69 years) (F:M ratio: 1.24:1)], 40 were primary, 11 metastatic, 4 recurrent, and 1 indeterminate. Carcinoma not otherwise specified (6), malignant not otherwise specified (5), differentiated thyroid cancer (3), atypical (2), suspicious for neoplasm/malignancy (2), and unsatisfactory (4) constituted the false negative diagnoses. Smear cellularity and cell distribution were variable. Malignant cells were mostly epithelioid, but accompanied by pleomorphic, spindle, and multinucleated forms. Differentiated thyroid carcinoma was diagnosed in 5 cases. Background necrosis and/or inflammation were common (80%). Three false positive cases included 1 example each of melanoma, Hürthle cell carcinoma, and metastatic squamous carcinoma. Where follow-up was possible, 91% of patients were dead of disease.

A FNA diagnosis of ATC/probable ATC was made in 34 out of 56 cases (sensitivity: 61%). Problematic cases were associated with incomplete sampling, lack of cell block immunostaining, uncertainty of anatomic site, and overlapping morphologic features with non-ATC malignancies.