Inflammatory pseudotumor of the thyroid gland showing prominent fibrohistiocytic proliferation. A case report.

Inflammatory pseudotumor of the thyroid gland (IPT) appears to be exceedingly rare. Histologically, 14 previously reported cases demonstrated plasma cell granuloma variant. We report here an IPT showing a predominantly fibrohistiocytic proliferation that occurred in a 75-year-old Japanese woman. Histologically, the lesion was characterized by haphazardly arranged spindle cells, histiocytes having foamy cytoplasm containing intracytoplasmic brown pigments, and small lymphocytes. Immunohistochemical study demonstrated that the spindle cells were vimentin+, desmin-, muscle-specific actin+, cytokeratin-, endomysial antibody-, anaplastic lymphoma kinase-, CD34-- CD68+/-, CD99-, cyclin D1-, bcl-2-, and antifollicular dendritic cell antibody-. IPT showing a predominant fibrohistiocytic proliferation should be differentiated from various nonneoplastic or neoplastic disorders showing spindle cell proliferation and/or exuberant fibrosis. They include Riedel's thyroiditis, fibrous variant of chronic thyroiditis, papillary carcinoma with exuberant nodular fasciitis-like stroma, paucicellular variant of anaplastic thyroid carcinoma, and solitary fibrous tumor.