Tumor biology and incidence

Caused by sporadic mutation of the GNAS1 gene that encodes the alpha subunit of the stimulatory G protein (G1).

Lesion of bone that results in a haphazard mixture of immature fibrous tissue and small fragments of immature trabecular bone.

Age

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Gender

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Presentation

Angular deformities may result from defective mineralization of immature dysplastic bone
Affected bone becomes widened and adjacent cortical bone becomes thin
Occurs typically in adolescence
Prevalence of malignant transformation (chondrosarcoma or osteosarcoma) is about 0.4 %

Physical findings

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Plain films

Appearance of lesions is variable
Intramedullary diaphyseal lesion that blends with thin, expanded cortex
Hazy or ground glass lesion which may occur with angular deformity
Angular deformity often present at the level of lesion
Active lesion may progress in size and deformity
Cystic lesion: radiolucent with a reactive rim, no trabeculae, normal cortical thickness
Pagetoid lesion: trabecular pattern which is denser than normal bone
Shepard's crook deformity (a favorite on OITE): extensive involvement of proximal femur results in a characteristic varus deformity that resembles a Shepard's crook

Site
Rib is involved most often but any bone can be involved
Also seen in proximal femur, proximal femur, mandible

Size

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Tumor effect on bone

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Bone response to tumor

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Matrix

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Cortex

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Soft tissue mass

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Bone scan

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CT Scan

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MRI

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Differential Diagnosis

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Natural history

Progression of the disease is often erratic
Monostotic lesions: good prognosis
Polyostotic lesions: tend to remain more active or aggressive, occasionally undergo malignant transformation to osteosarcoma or fibrosarcoma

Pathology

Irregular collection of immature woven trabeculae surrounded by abundant fibroblastic proliferation of fibrous tissue and immature woven trabeculae
Appearance likened to alphabet soup with C-, O-, and U-shaped trabeculae

Diagnosis and treatment

Associated conditions
Albrights Syndrome
Precocious puberty and café au lait pigmentation in females
Monostotic disease usually has no symptoms
Polyostotic disease usually presents earlier and may be unilateral or widespread, affecting long bones, hands, feet, and pelvis
Prevalence of malignant transformation (chondrosarcoma or osteosarcoma) is about 4%

Complications

Pathologic fracture
Bone becomes deformed by multiple stress fracture which may eventually lead to pathologic fracture
Painful stress fractures are especially common in femoral neck
Although dysplastic bone heals at normal rate after fracture, the resulting callus is also dysplastic, and the disease persists

Recommended Reading

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