PAUCICELLULAR VARIANT OF ANAPLASTIC THYROID-CARCINOMA - A MIMIC OF RIEDELS THYROIDITIS

Anaplastic thyroid carcinomas usually pose no problems in histologic d iagnosis because of the obvious invasive growth, high cellularity, and frank anaplasia. Two cases of a variant of anaplastic thyroid carcino ma with peculiar gross and histologic features closely mimicking those of Riedel's thyroiditis are described in this report. The clinical fe atures were no different from those of the usual anaplastic thyroid ca rcinomas: occurrence in elderly subjects, presentation with rapidly en larging neck mass associated with compression symptoms, and rapidly fa tal outcome. The tumors were infiltrative, hard, fibrotic masses that partly or completely replaced one lobe of the thyroid, and extended to perithyroid tissues. Histologically, they were predominated by acellu lar fibrous or infarcted tissue with central dystrophic calcification, as well as hypocellular foci comprising mildly atypical spindle cells intermingled with collagen and small lymphocytes. Both cases showed p ermeation and plugging of the arteries by tumor, Lymph node metastasis was documented in one case, The spindle cells were positive for epith elial membrane antigen in both cases, and cytokeratin in one. The qual ifying term ''paucicellular variant'' accurately describes this uncomm on morphologic variant of anaplastic thyroid carcinoma, It is importan t to recognize this variant so as not to mistaken it for Riedel's thyr oiditis, which is a reactive condition with a very favorable prognosis . The distinguishing features are as follows: presence of infarction, atypical cells in at least some areas, atypical spindle cells oblitera ting large blood vessels, and immunoreactivity for epithelial markers.