Papillary carcinoma of the thyroid with exuberant nodular fasciitis-like stroma.

We describe a rare case of papillary carcinoma with extensive proliferation of stromal cells. The stromal cells were immunocytochemically positive for vimentin, alpha-smooth muscle actin and desmin, but negative for cytokeratin, epithelial membrane antigen, S-100, thyroglobulin and CD34. These results and the ultrastructure of the stromal cells, which exhibited the characteristics of both fibroblasts and smooth muscle cells, indicated an origin from myofibroblasts. We conclude that myofibroblastic proliferation may contribute to the stromal response in the slow growth of the papillary carcinoma.