Paucicellular variant of anaplastic thyroid carcinoma: report of two cases.

The paucicellular variant of anaplastic carcinoma is an infrequent type of thyroid tumor. It was described as a tumor characterized by very low cellularity and prominent fibrosis, probably secondary to extensive infarction. These features could lead to an erroneous diagnosis of Riedel's thyroiditis. In this paper, we report the clinical and pathological features of two new cases of this unusual entity. Tumor cells were negative for thyroglobulin immunostaining and positive for keratins and p53. Although the number of reported cases is small, the cumulative data on these two cases and the previously reported ones lead us to suggest that the paucicellular variant may occur in younger patients than the conventional anaplastic thyroid carcinoma and that the tumor may be associated with a less aggressive tendency to local progression and metastasis.