Anaplastic (sarcomatoid) carcinoma of the thyroid gland.

Anaplastic (sarcomatoid) carcinomas of the thyroid gland (ACT) seem to be decreasing in frequency. During the past 45 years, a threefold to fourfold diminution in the number of ACT cases has been seen internationally. Despite this fact, these tumors continue to pursue a lethal course in virtually all cases. Clinically, ACT are characterized by rapidly growing neck masses, often in patients who have had long-standing thyroid nodules or goiters. Microscopically, ACT shows a mixture of fusiform and pleomorphic tumor cells, with widespread permeation of surrounding cervical tissues. Osteoclast-like giant cells, intranuclear cytoplasmic invaginations, and divergent differentiation into chondro-osseous, vascular, or myogenic tissues also may be observed in ACT histologically. The common presence of admixed foci of differentiated thyroid cancer in ACT suggests that the latter neoplasm arises through the mechanism of "clonal evolution." Keratin is observed immunohistologically in roughly 80% of cases, further supporting this contention. Because the incidence of ACT has inversely paralleled increasing use of thyroid surgery in general, it may be hypothesized that early removal of hyperplastic or neoplastic thyroid tissue aborts the previously mentioned "dedifferentiation" phenomenon.