Nonossifying fibroma

Tumor biology and incidence

Common benign lesion that is latent or active; rarely aggressive
Likely at one time a fibrous cortical defect that continued to enlarge
Approximately 5% of all investigated benign bone tumors
Almost indistinguishable from fibrous cortical defects, with exception of size and the extent of metaphyseal involvement
- Share common clinical presentation, histology, natural history, and pathology
- Commonly referred to collectively as metaphyseal fibrous defects
Etiology remains unknown
Theories of etiology
- Tumors develop from mature marrow, with complete absence of osseous metaplasia
- Tumors represent manifestation of local disturbance at epiphyseal plate; eventually replaced by bone

Age

First three decades of life

Gender

M:F = 2:1

Presentation

Usually asymptomatic; however, can be painful
More commonly symptomatic than a fibrous cortical defect
Occasionally it can present as a pathologic fracture, especially in active adolescents

Physical findings

Often no physical findings associated with lesion
Pain can occasionally be elicited on physical exam in area of defect
May be slight swelling if lesion is large enough

Plain films

Site

Most commonly affects tibia and femur
Lesion expands into medullary cavity in metaphysis
Long axis of lesion usually parallel to long axis of bone

Size

Usually 3 cm or more

Tumor effect on bone

Can have a scalloped border on x-ray with continued growth
Risk of fracture depends on extent of cortical thinning and size of lesion

Bone response to tumor

Usually thin, sclerotic, often lobulated margin

Matrix

May be multiloculated

Cortex

Cortex is slightly expanded and thinned
Cortical expansion of bone may occur over surface of lesion
Extensive periosteal reaction may indicate prior pathologic fracture

Soft tissue mass

None

Bone scan

Three-phase bone scan can detect if lesion is active, healing, or inactive based on the intensity of the uptake on the scan
- Intense hyperemia and positive scan may suggest associated fracture
- Mild hyperemia and moderate bone uptake present during healing phase
- Normal scans obtained when lesion is quiescent or has fully healed
Bone scans not routinely ordered to diagnose or manage metaphyseal fibrous defects
- Use may be indicated in cases where classic radiographic features are absent

CT scan

May be used to verify exact size of lesion, identify subtle changes in lesion size over time, and determine presence of cortical integrity or pathologic fracture

MRI

Can identify lesion with cystic components
May be useful to present a more detailed assessment of local osseous and soft-tissue response.
Some variation in appearance has been described on T2-weighted images
- T2 variations in high signal areas tend to stem from the varied amounts of hypercellular fibrous tissue, hemosiderin, hemorrhage, collagen, foamy histiocytes, and bone trabeculae present in a given lesion
All lesions tend to have decreased signal on T1-weighted images

Differential Diagnosis

Fibrous cortical defect (includes a differential diagnosis of osteoid osteomas, intracortical abscesses, stress fractures, and intracortical osteosarcoma)
Aneurysmal bone cyst
Chondromyxoid fibroma
Fibrous dysplasia
Desmoplastic fibroma

Variant is multiple nonossifying fibroma, which is associated with neurofibromatosis, von Recklinghausen's disease, and Jaffe-Campanacci syndrome (multifocal nonossifying fibromas with extraskeletal congenital anomalies, such as café-au-lait spots, mental retardation, hypogonadism, cryptorchidism, ocular anomalies, and/or cardiovascular malformations).

Natural history

Benign, self-healing lesion
Most undergo spontaneous regression, usually starting at the end of adolescence (as in fibrous cortical defects) and disappear by age 20-25
- Due to accelerated rate of skeletal maturation, females tend to encounter earlier regression
Average time from diagnosis to complete spontaneous regression: 29-52 months for fibrous cortical defects, longer for nonossifying fibromas (due to larger size)
Regressive phase characterized by trabecular bone growth beginning at periphery of the lesion and progressing centrally, until bone is fully reconstituted
Malignant transformation virtually 0%

Pathology

Identical to fibrous cortical defect

Gross: Cortex is replaced with fibrous tissue with granular appearance; may have yellow areas
Microscopic: Spindle cell proliferation with storiform arrangement
Multinucleated giant cells and abundant iron
Foamy histiocytes may be seen
Usually no bone formation

Diagnosis and treatment

Often definitively made based on patient's history, physical examination, and plain radiographs
If no symptoms, observation with serial radiographs recommended
- If size and appearance remain unchanged 3 months later, follow patient with serial radiographs every 6 months to 1 year until regression occurs or symptoms develop
- Goal of serial radiographs: Prevent pathologic fracture
Screening for multiple lesions indicated if patient presents with history consistent with neurofibromatosis, Jaffe-Campanacci syndrome, or pathologic fracture
A limited skeletal survey to rule out multifocal lesions should include AP radiographs of both tibias, fibulas, femurs, and humeri
Protected weight-bearing is controversial due to resultant adjacent disuse osteoporosis
Treat patient surgically with curettage and bone graft (possibly also with internal fixation) if there is:
- Atypical location or a questionable diagnosis
- Intractable pain
- Pathologic fracture
- Impending pathologic fracture (defined as greater than 50% involvement of the diameter of the femur or tibia)
If fracture occurs in child, may be best to allow fracture to heal prior to determining treatment; fracture may induce remodeling and healing of lesion

Complications

Pathologic fracture
- Generally occurs with larger lesions
- Tends to be non-displaced and through the lesion; managed with casting
- More common in the lower extremities: nearly 90% of all fractures, with almost 50% occurring in distal tibia
- Biopsy, curettage, and bone grafting recommended following fracture healing
- If displaced fracture occurs, intramedually nailing not contraindicated to achieve internal fixation