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Desmoplastic fibroblastoma (collagenous fibroma) is a rare, benign soft tissue that occurs in the subcutaneous tissue or skeletal muscle. It was first described by Evans in 1995. It affects mainly adult males. It do not recur and do not metastasize. Histologically, it is characterized by densely collagenous, predominantly stellate-shaped fibroblasts exhibiting bland cytological features. We report a case of desmoplastic fibroblastoma arising in the left buttock. A 41-year-old man presented with a left buttock mass for 5 months. Physical examination showed a nontender, firm mass on the left buttock. MR imaging showed a fibrous nodular tumor in the left lateral buttock. The clinical impression was desmoid fibromatosis. Wide excision was done. Gross examination showed a well circumscribed tumor, measuring 5.5×3.0×3.0 cm in size. The cut surface was grayish white and rubbery to firm in consistency. Microscopically, the tumor was composed of spindle or stellate-shaped cells with entirely patternless architecture. Tumor cells were distributed in a dense, hyalinized collagenous or myxoid stroma. The nuclei had delicate chromatin and small nucleoli. The cellular density was low. There was no nuclear atypia. No necrosis and mitotic figure were found. On the immunohistochemical stain, tumor cells were negative for smooth muscle actin, desmin, CD34, S-100 protein, neurofilament, and epithelial membrane antigen, Five months after surgery, the patient was well with no local recurrence. The important differential diagnostic consideration is desmoid fibromatosis. Although desmoplastic fibroblastoma histologically resembles desmoid fibromatosis, it is less cellular and have no long fascicular arrangement. The tumor margin is well circumscribed.
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