Background: Desmoid tumor is a rapidly growing tumor. It may be located in a variety of anatomic locations
such as abdominal, intra-abdominal and extra abdominal. However we have found only five cases in anterior mediastinum in the literature.
Case Report: A 55-year-old woman presented with history of dyspenea. Chest X-ray and computed tomography(CT) on admission showed a 30cm mass in the anterior mediastinal space, compressing the lungs
and adhering to chest wall, aorta and pericardium without any invasion. A diagnosis of benign
peripheral nerve sheath tumor was established by a CT-guided percutaneous needle biopsy. The mass was resected. The tumor was composed of bland spindled to stellate cells in a myxoid to highly collagenous stromal background. Tumor cells were positive for vimentin and negative for actin, EMA, CD34, cytokeratin, and S-100 protein. Pathological and immunohistochemical analyses confirmed a diagnosis of Fibromatosis. The patient has no recurrence 19 months after his operation.
Conclusions: Mediastinal fibromatosis is a rare tumor especially in the anterior mediastinum, which can mimic many benign and malignant tumors. Pathologist should consider it in differential diagnosis of
any bland spindle cell tumor especially in core needle biopsy.

Keywords: Fibromatosis, Anterior, Mediastinum, desmoid tumor