Rare Benign Fibrous Bone Tumour
- Rare (under 1% of benign bone tumours) and histologically identical to non-ossifying fibroma: storiform fibroblasts, osteoclast-type giant cells and foamy macrophages
- Distinguished from NOF on clinical and radiological grounds: atypical site and an older, usually symptomatic patient, against the young, asymptomatic, metaphyseal NOF that regresses
- The entity itself is contested: identical histology and shared MAPK-pathway mutations (KRAS, FGFR1) lead the current WHO classification to place it within the NOF spectrum. Know both positions
- Diagnosis of exclusion: correlate the clinical picture, imaging (well-defined lytic lesion, often with a sclerotic rim) and histology, ideally at a specialist bone-tumour centre
- Curettage (with grafting) or simple resection is usually curative; recurrence is not rare, so follow up. Rare malignant transformation, one case following prolonged stabilisation on denosumab
- 'Malignant fibrous histiocytoma' is a separate, malignant tumour, now largely reclassified as undifferentiated pleomorphic sarcoma
- “BFHB = NOF histology in the wrong place (pelvis/axial/diaphysis-epiphysis) and the wrong patient (older, symptomatic)
- “Giant cell tumour is the critical mimic: H3.3 G34W immunostain positive in GCT, negative in BFHB/NOF
- “Recurrence: ~11% is quoted with no published denominator; the defining series had 3 of 8 recur. Follow up
Overview & Epidemiology
How rare. Benign fibrous histiocytoma of bone (BFHB) represents less than 1% of benign bone tumours (Wetterwald 2024, repeating the conventional figure). Most published experience is case reports and small series, and the largest defining series is just 8 cases (Clarke 1985).
Why the entity exists at all. NOF is one of the commonest lesions of the growing skeleton. A childhood NOF is an incidental, metaphyseal, self-limiting lesion that regresses, whereas the same histology in an adult at an atypical site is symptomatic, persists, and recurs after curettage in a notable minority. The label survives because the clinical behaviour differs even though the histology does not.
Whether that justifies a separate name is contested (see Pathology), but examiners still expect you to know the classical clinical distinction and to state both positions.
- Benign fibrous histiocytoma of bone
- Storiform fibroblasts + giant cells + foamy macrophages
- Non-ossifying fibroma
- Identical
- Benign fibrous histiocytoma of bone
- Older (adults)
- Non-ossifying fibroma
- Children/adolescents
- Benign fibrous histiocytoma of bone
- Atypical (pelvis/axial/diaphysis-epiphysis)
- Non-ossifying fibroma
- Metaphysis of long bones (eccentric, cortical)
- Benign fibrous histiocytoma of bone
- Usually symptomatic (painful)
- Non-ossifying fibroma
- Usually asymptomatic/incidental
- Benign fibrous histiocytoma of bone
- Benign; up to ~11% recur; rare transformation
- Non-ossifying fibroma
- Benign; regresses/heals spontaneously
A nomenclature warning. The historical term "malignant fibrous histiocytoma" is a separate, malignant tumour, now largely reclassified as undifferentiated pleomorphic sarcoma. It shares only a name with the benign entity on this page, and the two must not be conflated.
Pathology and Molecular Basis
What the lesion is made of. A storiform (cartwheel) proliferation of fibroblastic spindle cells with a variable admixture of osteoclast-type giant cells and foamy (lipid-laden) macrophages, the same triad as NOF. Haemosiderin, xanthoma change and reactive woven bone or osteoid/chondroid metaplasia at the periphery are common, and should not be over-read as matrix production by the tumour itself.
Immunophenotype and histogenesis. The histiocytic cells and foamy macrophages are CD68/CD163-positive, and the osteoclast-type giant cells are CD68-positive. The old "fibrohistiocytic" name reflects a discredited histiocytic origin theory: this staining is a lineage marker of the reactive component, not evidence that the tumour is histiocytic, and the neoplastic cell is the fibroblastic spindle cell. There is no specific defining marker, so immunohistochemistry is used mainly to exclude mimics.
Giant cell tumour of bone carries the H3F3A (H3.3) G34W mutation, so the H3.3 G34W immunostain is positive in GCT and negative in BFHB/NOF. It is the single most useful marker for that critical distinction.
The driver. NOF, and by histological identity BFHB, is a MAPK-pathway-driven neoplasm: activating hotspot mutations in KRAS, FGFR1 or NF1 are found in about 81% of NOFs (Baumhoer 2019). The puzzle is behavioural. The same driver underlies a childhood lesion that regresses spontaneously and an adult lesion that persists and recurs, and why regression fails in the adult or atypical setting is not known: the sequencing data describe the engine, not the brakes.
Why the entity is contested. The shared molecular basis, together with the identical histology, has led many, and the current WHO classification, to regard BFHB as within the NOF spectrum (an "atypical" or symptomatic, atypically sited NOF) rather than a truly separate entity.
Clinical Presentation
The typical story. An adult with a painful bone lesion. Pain without a complicating fracture is the feature that most consistently separates BFHB from the incidental, asymptomatic childhood NOF (Clarke 1985). The presentation is otherwise non-specific: a gradually progressive deep ache, sometimes with local swelling or tenderness, occasionally unmasked by a pathological fracture through a larger lesion.
Where to look. Any bone can be involved, but the atypical distribution is the point. BFHB is not confined to the metaphysis of a growing long bone, and epiphyseal and diaphyseal long-bone sites, the pelvis, the axial skeleton and ribs are all described (Clarke 1985). A "NOF-like" radiographic lesion in an adult, at one of these sites, with pain, should trigger the BFHB differential rather than reassurance.
What it is not. It does not present with systemic symptoms, a soft-tissue mass or rapid growth. Those features point away from BFHB and toward the malignant mimics.
Investigations & Biopsy
The work-up. Plain radiographs characterise the lesion. MRI defines local extent for surgical planning, and CT is useful in the pelvis and axial skeleton, where radiographs under-show the lesion. There is no role for routine staging beyond local imaging in a histologically confirmed benign lesion.
When to think again. Any aggressive feature (cortical destruction, a soft-tissue mass, rapid growth) should prompt re-biopsy and sarcoma-unit referral rather than reassurance or a wider local work-up, because transformation, though rare, is real (Wetterwald 2024).
Exclude the metabolic mimic. Clarke's series noted that hyperparathyroidism may need to be excluded biochemically, because a brown tumour can mimic a giant-cell-rich fibrous lesion histologically. A calcium, phosphate and parathyroid hormone level is cheap insurance before labelling an adult with an atypical lytic lesion.
A diagnosis of exclusion. The histology overlaps with several giant-cell-rich and fibrous lesions, so the clinical setting (older, symptomatic patient; atypical location), the imaging and the histology must be correlated, ideally at a specialist bone-tumour centre. Giant cell tumour, NOF, fibrous dysplasia, aneurysmal bone cyst and chondromyxoid fibroma are the lesions to exclude.
Biopsy. The biopsy must be adequate and well planned: image-guided core biopsy at, or in consultation with, the treating bone-tumour centre, with the tract placed so it can be excised if the lesion proves to be something more significant, particularly giant cell tumour. A "NOF-like" biopsy from an aggressive-looking lesion is a sampling problem until proved otherwise.
Imaging Features and the Radiographic Differential
BFHB on imaging. A geographic, well-defined lytic lesion, usually with a thin sclerotic rim (a marginated, non-aggressive pattern), eccentric or central, and sometimes mildly expansile or multiloculated. There is no matrix mineralisation and no aggressive periosteal reaction unless there is a pathological fracture. MRI is non-specific (low-to-intermediate T1, variable T2, enhances).
The imaging differential. Each mimic has a radiographic tell:
- Giant cell tumour, the key one: epi-metaphyseal, subarticular (abutting the joint), eccentric and purely lytic, with an ill-defined, non-sclerotic margin, in a skeletally mature adult. The lack of a rim and the subarticular site favour GCT.
- Non-ossifying fibroma: metaphyseal, cortically based, with a well-defined sclerotic scalloped margin, "bubbly", in a child, and it regresses.
- Fibrous dysplasia: medullary, with a "ground-glass" matrix, often long-segment with bowing.
- Aneurysmal bone cyst: markedly expansile, with fluid-fluid levels.
- Chondromyxoid fibroma: eccentric and metaphyseal, lobulated, with a sclerotic scalloped margin.
Management
Treatment. Curettage (with bone grafting) or simple resection is usually curative, with grafting or fixation as needed for structural lesions.
Follow-up. Follow these patients up for local recurrence and, rarely, transformation. There is no evidence-based follow-up interval, so local protocols apply.
Aggressive, recurrent or transformed lesions. A recurrent or changing lesion deserves re-biopsy, and these lesions are managed at a sarcoma unit. Denosumab has been used for unresectable or recurrent disease and produced prolonged stabilisation in a reported case, but that same patient subsequently underwent malignant transformation. It is therefore a specialist decision under oncological supervision, not a routine option, and continued surveillance is required whatever the initial response.
Complications, Recurrence & Prognosis
Local recurrence is the defining complication, and two numbers circulate. The widely quoted "up to 11%" is repeated from earlier literature by a single case report without a stated denominator or source cohort (Wetterwald 2024), while the defining series recorded 3 recurrences among 8 patients (Clarke 1985). Neither is a firm consent figure; read together they support the honest statement that recurrence after curettage is not rare.
Malignant transformation is rare but documented: the Wetterwald case transformed after prolonged stabilisation on denosumab. Whether denosumab played any causal role cannot be determined from one case.
Pathological fracture can present or complicate larger lesions, particularly in weight-bearing bone.
Prognosis. With complete curettage or resection the outlook is excellent, and no metastasis was recorded in the defining series.
Mnemonics & Memory Aids
STORIFORM
Hook:STORIFORM: Storiform/giant cells/foamy macrophages, aTypical site+older, exclude Other lesions, Infrequent, curette/resect, recur ~11%, not Malignant FH.
RIMImaging Signature
Hook:RIM: sclerotic Rim, atypIcal site, no Matrix.
GROWDifferential You Must Exclude
Hook:Before calling it BFHB, make the mimics GROW: GCT, Rest of giant-cell-rich lesions, NOF, and Watch for a brown tumour.
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“A biopsy of a painful pelvic lytic lesion in an adult shows storiform fibroblasts with giant cells and foamy macrophages. What is the diagnosis and how do you manage it?”
What it is
- Rare benign fibrous bone tumour (under 1% of benign bone tumours)
- Histology = storiform fibroblasts + osteoclast-type giant cells + foamy macrophages (like NOF)
- Controversial entity
Distinguished from NOF
- Atypical site: pelvis, axial skeleton, long-bone diaphysis/epiphysis
- Older, usually symptomatic (painful) patients
- NOF = young, asymptomatic, metaphyseal, regresses
Behaviour & management
- Benign; curettage (+graft) or simple resection usually curative
- Recurrence not rare (~11% quoted, no published denominator) - follow up; rare malignant transformation
- WHO now places it within the NOF spectrum (shared KRAS/FGFR1) - not a distinct entity
- Denosumab reported for recurrent disease: stabilised, then transformed - specialist decision only
- Diagnosis of exclusion; don't confuse with malignant fibrous histiocytoma (UPS)
Evidence & Key Studies
Recurrent benign fibrous histiocytoma of bone with later malignant transformation
- A SINGLE CASE REPORT - of a pelvic benign fibrous histiocytoma of bone that recurred, stabilised for a prolonged period on denosumab, and then underwent malignant transformation.
- Describes the entity as rare (less than 1 percent of all benign bone tumours) and explicitly as CONTROVERSIAL, with the storiform fibroblastic histology, osteoclast-type giant cells and foamy macrophages.
- The often-quoted relapse figure of up to 11 percent is cited BY this report from earlier literature - it is not a rate this paper measured, and no denominator or source cohort is given with it.
- Denosumab produced long-lasting disease stabilisation before transformation occurred; whether that sequence is causal or coincidental cannot be determined from one case.
- A single patient: it establishes that malignant transformation can happen, and nothing about how often.
The rarity (less than 1% of benign bone tumours), the storiform fibroblastic histology with osteoclast-type giant cells and foamy macrophages, the usual cure by curettage/simple resection with up to about 11% relapse, and the rare malignant transformation come from the cited Wetterwald report. The defining clinical distinction from NOF (adults, painful, atypical sites) and a real recurrence denominator (3 of 8) come from the Clarke 1985 series, and the shared KRAS/FGFR1 MAPK-driver basis from Baumhoer 2019. The histological identity with non-ossifying fibroma and the clinical distinction (atypical site, older/symptomatic patients), the status as a diagnosis of exclusion, and the separation from 'malignant fibrous histiocytoma' (undifferentiated pleomorphic sarcoma) are standard, well-established teaching. The cited source is a single case report, so it establishes that malignant transformation and a denosumab response can occur, and nothing about how often; the 11 percent relapse figure is one that report quotes from earlier literature rather than measures, and no cohort with a stated denominator supporting it was retrieved - so it is given here with that caveat and should not be quoted as a firm consent figure. No trial of any treatment in this entity exists, and no evidence-based follow-up interval or duration was found. See also non-ossifying fibroma, giant cell tumour of bone, fibrous dysplasia, aneurysmal bone cyst, chondromyxoid fibroma and, for the malignant tumour whose old name causes the confusion, undifferentiated pleomorphic sarcoma.