A case of desmoplastic fibroma of bone with CTNNB1 point mutation.

Desmoplastic fibroma of bone (DFB), a bone tumor, is considered to be an osseous counterpart of desmoid-type fibromatosis (DF). Herein, we report a case of DFB with CTNNB1 point mutation. The 5-year-old male patient had complained of trismus and pain in the jaw. Magnetic resonance imaging revealed a mass in the left mandible. Radical treatment involved surgical resection. Microscopically, the lesion consisted of a bundle-like proliferation of uniform spindle-shaped cells with abundant collagenous stroma, which resembled DF. Immunohistochemical analysis revealed intranuclear accumulation of β-catenin in the tumor cells. Based on clinical and histologic analysis, we diagnosed the patient as having DFB. We examined the CTNNB1 and APC sequence and found an A-to-G transition at codon 41 of CTNNB1; i.e., Thr was substituted by Ala. Our findings suggest that the dysregulation of Wnt/β-catenin signaling pathway is related to the tumorigenesis of some cases of DFB. This hypothesis indicates that there are some cases of DFB in which nuclear positive expression of β-catenin is useful for diagnosis.