Anaplastic thyroid carcinoma: Clinicopathologic and immunohistochemical study of 144 cases with special emphasis on the spectrum of histologic features.

We have studied 144 patients with anaplastic thyroid carcinoma (ATC) to evaluate the utility of a select panel of immunohistochemical stains for diagnosis and to better define their spectrum of histologic features. There were 97 women and 47 men, aged 48 to 103 years (mean: 67.8). The tumors were seen against a background of papillary thyroid carcinoma in 33 cases, follicular thyroid carcinoma in 28, poorly differentiated carcinoma in 5, and thyroid nodular disease in 31. Histologically, two basic patterns of growth were observed, one predominantly composed of spindle and pleomorphic cells, and one composed of round epithelioid cells. Several unusual morphologic features were also observed focally that can introduce difficulties for diagnosis, including tumors with squamous differentiation (47 cases; 32.6%), clear cells (17; 11.8%), paucicellular (17; 11.8%), inflammatory (16; 11%), pseudoangiosarcomatous (11; 7.6%), rhabdoid (9; 6.2%), and osteoclastic giant cells (8; 5.5%). Immunohistochemical staining was performed in 107 cases: AE1/AE3 cytokeratin stains were positive in 63 cases (58.8%) and CK8/18 in 46 (37.3%); PAX8 was positive in 35 cases (32.7%); TTF1 was only focally positive in 4 cases (3.7%); p40 was positive in 22 cases (20.5%), mostly in the areas of squamous differentiation. Strong CD10 positivity was also seen in 97 cases (90.6%). Our study shows that anaplastic carcinoma has the potential for simulating a large variety of tumors and underscores the difficulties for making the diagnosis using immunohistochemistry; cytokeratins, PAX8 and TTF1 were of limited utility and showed inconsistent results emphasizing the importance of clinicopathologic correlation for the diagnosis.