Desmoplastic Fibroma of Bone

Not clinically relevant

Unknown

Due to its high collagen content, desmoplastic fibroma is a firm, creamy-white tumor with a whorled architecture. The tumor border is well defined and has scalloped margins, even when it extends into soft tissue.

Desmoplastic fibroma has an infiltrative growth pattern and typically consists of a tumor of low cellularity composed of fascicles of bland nondescript spindle cells set in a collagenous matrix. Mitoses are scant. Necrosis is not present. The fibroblastic spindle cells show variable immunoreactivity for SMA; β-catenin can be present, most commonly in the cytoplasm. The differential diagnosis includes fibrous dysplasia, low-grade central osteosarcoma, low-grade myofibroblastic sarcoma, myoepithelial tumors, follicular dendritic cell tumors, and synovial sarcoma.

Not clinically relevant

Prognosis and prediction are difficult to predict because so few cases have been reported. Desmoplastic fibroma is a slowly progressive and locally aggressive tumor, which may recur after curettage or intralesional excision.

In view of recent advances in molecular pathology allowing classification of bone tumors more objectively, the diagnosis of desmoplastic fibroma should be one of exclusion. In particular, fibrous dysplasia and low-grade central osteosarcoma should be excluded by molecular tests for GNAS single-nucleotide variants and MDM2 amplification, respectively. There are reports of two cases of desmoplastic fibroma in bone revealing a CTNNB1 mutation: one p.Thr41Ala and one p.Ser45Phe alteration. Both cases exhibited nuclear β-catenin immunostaining. Previously, CTNNB1 mutations were not detected in a series of 13 cases.

Essential: a bland spindle cell lesion in bone; collagenous matrix.

Desirable: exclude fibrous dysplasia by showing the absence of GNAS mutations; exclude low-grade central osteosarcoma by showing the absence of MDM2 amplification.