Nonconventional papillary thyroid carcinomas with pleomorphic tumor giant cells: a diagnostic pitfall with anaplastic carcinoma.

The presence of pleomorphic tumor giant cells in thyroid carcinomas of follicular cell origin is always worrisome for the pathologist as they first of all refer to anaplastic carcinoma, one of the most aggressive human malignancies. However, non-anaplastic pleomorphic giant cells are well described in other thyroid diseases, most often benign. In this paper, we describe four cases of papillary thyroid carcinoma displaying pleomorphic tumor giant cells with features that differ from those of anaplastic carcinoma. Pleomorphic giant cells were admixed with the underlying thyroid carcinoma and constituted from 5% to 25% of the tumor. Cytologically, they had an abundant eosinophilic cytoplasm with large and irregular nuclei. Compared to pleomorphic giant cells of anaplastic carcinoma, they reproduced the growth pattern of the underlying carcinoma, had a low mitotic index without necrosis or inflammation, and were reactive with thyroglobulin and thyroid-specific transcription factor-1 and strongly and diffusely positive for cytokeratin AE1/AE3. After 16-84 months of follow-up, patients are relapse-free and still alive. These cases show that pleomorphic tumor giant cells arising in papillary thyroid carcinomas do not always represent dedifferentiation and progression to anaplastic carcinoma. Distinction among these processes is critical as their treatment and prognosis are very different.