Papillary carcinoma of the thyroid gland forming a myofibroblastic nodular tumor: report of two cases and review of the literature.

We encountered two unique cases of papillary carcinoma of the thyroid gland forming fibroma-like nodular tumors. The two cases showed well-defined nodules composed largely of fibromatosis-like stroma with small foci of papillary carcinoma. This type of tumor is relatively rare and has been reported with the name of 'papillary carcinoma of the thyroid with fibromatosis-like stroma or nodular fasciitis-like stroma'. The stromal cells have had myofibroblastic features in all reported cases, including the present cases. The pathogenesis of this tumor is still unknown. We must be careful to differentiate this type of thyroid tumor from fibrosarcomatous anaplastic carcinoma, particularly in the intraoperative consultation. We speculate that this unique tumor is an autonomous proliferation of stromal cells, based on the immunohistochemical analyses of early stromal changes in usual-type papillary carcinoma of the thyroid. The present names of this disease may not represent the proper features of this tumor because all the reported cases formed fibroma-like nodules. We propose the name 'papillary carcinoma forming myofibroblastic nodular tumors'.