Desmoplastic Fibroma of Bone

Desmoplastic fibroma is the rarest primary bone tumour, less than 0.3% of all primary bone neoplasms, with over 300 cases in the literature as case reports and small series. It is the intraosseous counterpart of the extra-abdominal desmoid tumour (aggressive fibromatosis): histologically benign and never metastasising, but locally aggressive, with a propensity for cortical destruction and local recurrence that mirrors a low-grade malignancy.

Who. A patient aged 10-30, peaking in the second decade, though the range runs from childhood to the elderly. Males and females are affected roughly equally.

Why it matters. The two errors run in opposite directions. Misdiagnosis as fibrosarcoma or low-grade osteosarcoma leads to unnecessary radical surgery or chemotherapy; under-treatment with curettage alone leads to recurrence. Morbidity comes from local destruction (progressive bone resorption, cortical expansion, pathological fracture) and from the surgical complications of the wide resection the tumour demands.

What it is. A clonal neoplastic proliferation of myofibroblasts producing abundant type I and type III collagen. Morphologically and biologically it is the intraosseous counterpart of soft-tissue desmoid tumour (extra-abdominal fibromatosis), but the resemblance is histological and behavioural rather than molecular, and that distinction is the examinable point.

Histology. Uniform, small, inactive-appearing spindle cells sit sparsely in a dense collagenous stroma: low cellularity, abundant collagen. There is no nuclear pleomorphism, hyperchromasia or prominent nucleoli, mitoses are rare or absent, and there is no necrosis and no osteoid. The picture overlaps with soft-tissue desmoid tumour and mimics low-grade fibrosarcoma; the absence of atypia and of significant mitoses is what separates it from sarcoma.

The molecular biology. When Hauben and colleagues studied 13 cases, sequencing for activating beta-catenin mutations succeeded in 6 and none carried a CTNNB1 mutation; nuclear beta-catenin was demonstrable in only 1 of 13. The authors concluded that the Wnt/beta-catenin pathway does not have the same essential role here that it has in desmoid-type fibromatosis, so do not expect the desmoid genotype and do not rely on nuclear beta-catenin to make the diagnosis. Occasional reports describe APC alterations, but a 23-year national database search found no co-occurrence with colon cancer, and a routine FAP work-up is not indicated on the basis of a desmoplastic fibroma alone. Trisomy 8 and trisomy 20 have been reported in some cases: non-specific, but supporting the clonal neoplastic nature of the lesion.

How it grows. The lesion begins in the medullary cavity and expands slowly but relentlessly from within, thinning the cortex to produce the characteristic expansile lytic appearance. It produces no osteoid or matrix, so there is no mineralisation. Advanced lesions may breach the cortex and extend into soft tissue, and there is no true capsule.

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Why it recurs. The margin is infiltrative: the tumour sends finger-like projections into the surrounding bone with no capsule and, unlike a benign cyst, no clear demarcation. Curettage therefore leaves pockets of tumour cells in the bony interstices, and because growth is slow a recurrence may take 1-5 years to become evident on radiographs. Adjuvants help, but they do not replace wide margins.

The archetypes. The presentations to hold in mind:

• A young adult with a painless mandibular swelling
• An adolescent with a lytic lesion of the distal femur
• A recurrent lytic lesion after curettage elsewhere

History. Pain is slowly progressive and often mild for months, and symptoms have been present for months to years by the time the diagnosis is made. A gradually enlarging mass is the other complaint, especially in the mandible or tibia, and mandibular lesions bring dental symptoms: loose teeth and difficulty chewing. In a weight-bearing bone a pathological fracture may be the presenting event.

Examination. A bony-hard mass, non-tender or mildly tender. There is no regional lymphadenopathy and there are no systemic features (weight loss, fever, malaise); the neurovascular examination is intact unless the spine or pelvis is involved. Assess the range of motion of an adjacent joint when the lesion lies near the articular surface.

Suspicion. Desmoplastic fibroma is so rare that it is almost never the first diagnosis considered for a lytic bone lesion. What should raise it is the combination: a young patient, a well-defined lytic lesion with internal trabeculation, no matrix mineralisation and no aggressive periosteal reaction. Include it in the differential for any trabeculated lytic metaphyseal lesion in a young patient, especially in the mandible or femur.

Radiographs. AP and lateral of the affected bone, with a panoramic dental view for mandibular lesions. The characteristic picture is a well-defined lytic metaphyseal lesion whose internal trabeculation gives a honeycomb (soap-bubble) appearance, with cortical thinning or expansion, no matrix mineralisation, no periosteal reaction unless the bone has fractured, and no soft-tissue mass in early lesions. Plain films are often suggestive but not diagnostic; advanced imaging is always needed.

The radiographic pattern narrows the differential:

MRI. T1, T2, STIR and post-gadolinium T1 fat-saturated sequences. The signal is low-to-intermediate on T1 and low-to-intermediate on T2, reflecting the dense fibrous collagen content, with homogeneous gadolinium enhancement. Most other lytic lesions (giant cell tumour, aneurysmal bone cyst, fibrous dysplasia) show high T2 signal because of their fluid or cellular content, so the low T2 signal is the distinguishing feature. MRI also maps extent within the medullary canal, cortical breakthrough, soft-tissue extension and the relationship to neurovascular structures and the joint.

CT. The best study for cortical integrity, the pattern of internal trabeculation and matrix characteristics: a lytic lesion with coarse internal bony septa, cortical thinning and no mineralised matrix. It defines the bony anatomy for resection and reconstruction planning, especially in the mandible and pelvis.

Biopsy. Core needle biopsy is preferred because it gives adequate tissue for histology and immunohistochemistry, and it is performed only after imaging is complete and the case has been discussed at a multidisciplinary bone tumour meeting. The tract must lie in the planned surgical field so that it is excised en bloc at definitive surgery. The lesion can be sampled inadequately, so ensure sufficient tissue for specialist review.

The language of resection. Every decision below is expressed in Enneking margin terms, so define them first. The margin is named by the plane of dissection relative to the tumour and its reactive zone.

The decision. Recurrence tracks the margin. Intralesional curettage recurs in 17-72% of cases; wide excision brings that to under 10%, which is why it is the gold standard, and why its morbidity must be justified by expert biopsy confirmation before proceeding. Marginal excision sits between, at an estimated 10-25%, and is acceptable when wide margins are anatomically impossible. Adjuvants improve local control after curettage but do not equal wide excision, and the evidence for them is limited.

Recurrence. Recurrence is the defining clinical challenge: unlike most benign bone tumours, desmoplastic fibroma recurs aggressively after intralesional treatment, and a recurrence signals inadequate initial margins. The time to recurrence ranges from months to years, which is the reason for the long surveillance schedule. A recurrence is managed by re-resection with wide margins and continued imaging surveillance, and each one may require progressively more extensive surgery, which is why the margins must be right the first time.

Desmoplastic fibroma never metastasises and never causes death from disease, but it causes significant morbidity from local recurrence and from surgical reconstruction. The prognosis for local control is overwhelmingly determined by the adequacy of the initial surgical margins.

Favourable. Wide excision with negative margins, a small lesion and an accessible site such as the tibia or fibula. Function after wide excision is good to excellent depending on the site: higher initial morbidity, best long-term control.

Unfavourable. Intralesional surgery, a large lesion with cortical breakthrough, a challenging site (pelvis, spine) and young age, which brings more growth-related morbidity. Curettage alone gives good function initially but recurrence is common; curettage with adjuvants recurs in 15-30% depending on the adjuvant, and both it and marginal excision give good functional outcomes.