Fibrous dysplasia (FD) is a benign fibro-osseous lesion resulting from replacement of normal bone marrow and trabecular bone with fibrous tissue and immature (woven) bone. It constitutes 70% of all fibro-osseous lesions and appears during skeletal development (childhood/adolescence). Monostotic form (70-80%) is much more common than polyostotic form. Temporal bone involvement occurs in 18% of monostotic FD and is the most common cause of temporal bone fibro-osseous lesions. The disease usually stabilizes by early adulthood but reactivation may occur with hormonal stimulation during pregnancy. External auditory canal narrowing (stenosis) is the most common symptomatic presentation, causing conductive hearing loss. McCune-Albright syndrome (polyostotic FD + cafe-au-lait spots + precocious puberty) is a rare but important association.
Age Range
5-30
Peak Age
15
Gender
Equal
Prevalence
Uncommon
FD results from a postzygotic somatic activating mutation in the GNAS1 gene (20q13). This mutation constitutively activates the Gs-alpha subunit — cAMP levels remain chronically elevated. Elevated cAMP disrupts osteoblastic differentiation in bone marrow stromal cells: disorganized immature woven bone and cellular fibrous tissue are produced instead of mature lamellar bone. Woven bone lacks the organized collagen fibers of normal lamellar bone — random collagen arrangement reduces mechanical strength but fracture risk is still low because fibrous matrix provides supportive role. The 'ground-glass' appearance on CT reflects the homogeneous intermediate density of this woven bone + fibrous tissue mixture — not as dense as pure bone (>800 HU), not as low as pure soft tissue (30-60 HU), showing an intermediate density (100-400 HU). This homogeneous intermediate density is not this prominent in any other bone pathology and is pathognomonic for FD. On MRI, the dominant fibrous content shows low-intermediate signal on T1 and T2 — high collagen/protein concentration restricts proton mobility and lowers T2 signal.
Homogeneous intermediate density (100-400 HU) bone expansion on CT — characteristic appearance of woven bone + fibrous tissue mixture. This density pattern is pathognomonic for fibrous dysplasia and is not this homogeneous and prominent in any other bone pathology. Clearly visible on bone window settings.
Homogeneous 'ground-glass' density — intralesional density ranges between 100-400 HU. This density is significantly lower than normal bone dense density (>800 HU) but higher than soft tissue (30-60 HU). The lesion appears homogeneous — internal cystic or calcified foci are rare.
Report Sentence
An expansile lesion with homogeneous ground-glass density in the temporal bone is identified, consistent with fibrous dysplasia; cortical integrity is preserved.
External auditory canal (EAC) stenosis — bone expansion with ground-glass density narrows the EAC lumen. Complete obliteration may also occur. This finding is the most common symptomatic complication of temporal bone FD and forms the basis for surgical indication.
Report Sentence
Significant stenosis of the external auditory canal due to bone expansion from fibrous dysplasia is observed (lumen diameter: ___ mm); evaluation for canaloplasty is recommended.
Low-intermediate signal intensity on T2-weighted images. This signal reflects the dense collagen content of the fibrous tissue component. Signal is significantly lower compared to fluid-containing lesions. Focal T2 hyperintensity may be seen in areas of cystic degeneration.
Report Sentence
On MRI, the lesion demonstrates low-intermediate signal intensity on T2-weighted sequences, consistent with the fibrous tissue content of fibrous dysplasia.
Bone expansion — the lesion expands the affected bone but cortical integrity is maintained. This feature confirms the benign process and differentiates from aggressive/malignant lesions (cortical break, periosteal reaction).
Report Sentence
An expansile ground-glass lesion in the temporal bone is identified with preserved cortical integrity; consistent with fibrous dysplasia and no evidence of aggressive pathology.
Variable enhancement on post-contrast T1 — ranges from mild to prominent. Enhancement degree is related to fibrous tissue vascularity and presence of cystic degeneration. Aggressive enhancement raises suspicion for malignant transformation.
Report Sentence
Mild-moderate enhancement of the lesion is observed on contrast-enhanced series; no aggressive enhancement pattern is detected.
Criteria
Single bone involvement. 70-80% of all FD cases. Temporal bone involvement is more common in this form.
Distinct Features
Isolated temporal bone lesion. Usually stable course. Surgery only in symptomatic cases (EAC stenosis, hearing loss). Low risk of malignant transformation (0.5%).
Criteria
Two or more bone involvement. 20-30% of all FD cases. Craniofacial involvement more common.
Distinct Features
Multiple bone involvement — temporal bone + sphenoid + maxilla common combination. Presents at younger age. May have more aggressive course.
Criteria
Polyostotic FD + cafe-au-lait spots + endocrinopathies (precocious puberty, hyperthyroidism). Rare (<3% of all FD).
Distinct Features
Requires endocrinological evaluation. Cafe-au-lait spots are 'coast of Maine' type (irregular border — different from NF1 'coast of California' smooth border). More extensive skeletal involvement.
Distinguishing Feature
LCH shows lytic destructive lesion (punched-out, no sclerotic rim); FD shows expansile ground-glass lesion with preserved cortical integrity. In LCH bone is destroyed, in FD bone expands.
Distinguishing Feature
Osteoblastoma usually shows expansile lytic lesion with sclerotic rim and heterogeneous density (internal calcification) is typical; FD shows homogeneous ground-glass density. Nidus and surrounding sclerosis may be present in osteoblastoma.
Distinguishing Feature
Metastasis shows destructive lytic lesion with irregular margins and cortical break; FD shows expansile ground-glass lesion with intact cortex. Age group differs — metastasis in elderly, FD in young.
Urgency
routineManagement
surgicalBiopsy
Not NeededFollow-up
12-monthTypical CT findings of temporal bone FD (ground-glass density + expansion + intact cortex) are diagnostic and biopsy is generally not required. Surgical indications: symptomatic EAC stenosis, conductive hearing loss, cosmetic deformity, and facial nerve compression. Surgery aims only at symptom control — complete resection is usually not possible and recurrence is common (20-30%). Malignant transformation risk is low (0.5% monostotic, 4% polyostotic) but MRI evaluation should be performed in case of rapid growth or increasing pain.
Fibrous dysplasia generally has a benign course. EAC stenosis leads to conductive hearing loss. Surgery is indicated for symptom control and canal stenosis. Malignant transformation is rare (0.5%) but osteosarcoma should be excluded in rapidly growing lesions.