Pseudosarcomatous fibromatosis (PSF), also known as nodular fasciitis and pseudosarcomatous fasciitis, is a rare, benign proliferation of fibroblasts and myofibroblasts. It is often misdiagnosed as sarcoma because of its characteristic rapid progression, leading to unnecessary and aggressive surgery. Here we report a case of PSF found in the masseter muscle in a 26-year-old female. The mass that could be misdiagnosed because of its rapid growth characteristics was surgically excised by an intraoral approach and diagnosed as PSF by histopathologic examinations.

Fibromatoses are benign fibroblastic proliferation, which forms infiltrating tumor-like mass. Their histologic features cannot reflect the biologic behavior; local recurrence and aggressive infiltration to the adjacent tissue make fibromatoses intermediate tumor, between benign and malignant. Infantile fibromatosis consists of desmoid-type tumor and lipofibromatosis, and mainly affects children under age of 10. While desmoid-type tumor is characterized by elongated mature fibroblastic proliferation, the fibroblasts in lipofibromatosis look more primitive and accompany adipocytes. Here, we report a case of infantile fibromatosis of 4 month old boy involving the tongue.