Fibrous dysplasia is a benign fibro-osseous lesion resulting from replacement of normal bone and bone marrow by fibrous tissue and immature woven bone. In the paranasal sinuses, it most commonly involves the maxilla and frontal bone. The monostotic form (70-80%) involves a single bone, while the polyostotic form (20-30%) involves multiple bones and may be associated with McCune-Albright syndrome (polyostotic fibrous dysplasia + café-au-lait spots + endocrine disorders). The disease typically begins in childhood and adolescence and tends to stop growing with skeletal maturation. The characteristic 'ground-glass' density on CT is pathognomonic and usually makes the diagnosis by itself. Malignant transformation risk is very low (0.5%) but increases in patients who have received radiotherapy.
Age Range
5-30
Peak Age
15
Gender
Equal
Prevalence
Uncommon
Fibrous dysplasia results from a postzygotic somatic activating mutation in the GNAS1 gene (20q13), which leads to constitutive activation of the Gsα subunit. Activated Gsα continuously stimulates adenylate cyclase and increases intracellular cAMP levels — this disrupts osteoblast differentiation and causes production of immature woven bone and fibroblastic stroma instead of normal lamellar bone. Because immature woven bone is not as organized as lamellar bone, it produces 'ground-glass' density on CT — homogeneous but lower density than normal cortical bone. The postzygotic and somatic nature of the mutation explains the mosaic distribution of the disease: in monostotic form the mutation occurs late, in polyostotic form early, leading to broader tissue involvement. In McCune-Albright syndrome, the mutation occurs very early in embryonic development and causes endocrine dysfunction in non-bone tissues (thyroid, pituitary, adrenal, gonads) as well.
The pathognomonic finding of fibrous dysplasia on CT is homogeneous ground-glass bone density (56-90 HU). This density reflects the homogeneous mixture of immature woven bone and fibrous stroma. It is significantly lower than normal cortical bone (>1000 HU) but higher than soft tissue (40-60 HU). When ground-glass density is observed together with bone expansion and cortical thinning, it largely makes the diagnosis and biopsy is not needed in most cases.
The pathognomonic finding of fibrous dysplasia on CT is homogeneous 'ground-glass' bone density. This density is significantly lower than normal cortical bone (>1000 HU) but higher than soft tissue (40-60 HU) — typical range is 56-90 HU. The homogeneous ground-glass appearance reflects equal distribution of immature woven bone and fibrous stroma. The lesion expands the bone by filling the medullary cavity (fusiform expansion) with cortical bone continuity maintained but possibly thinned. In paranasal sinuses, maxilla and frontal bone are most commonly involved. The lesion shows continuity with surrounding bone — there is no distinct border or capsule ('fusion with surrounding bone').
Report Sentence
A fibro-osseous lesion with bone expansion and homogeneous ground-glass density (__ HU) is observed in the maxilla/frontal bone; the appearance is consistent with fibrous dysplasia.
On bone window CT, fibrous dysplasia shows fusiform (spindle-shaped) expansion of the affected bone. The cortical bone is thinned but its continuity is maintained — this reflects the benign slow growth pattern. The border with surrounding bone is not distinct; the lesion 'blends' with normal bone (imperceptible transition). This feature distinguishes it from ossifying fibroma which shows distinct border and capsule. In paranasal sinuses, the expanding bone may narrow or completely obliterate the sinus cavity. Orbital wall involvement carries risk of proptosis, optic canal involvement risk of vision loss. Narrowing of bony foramina and canals (infraorbital canal, optic canal) should be evaluated for surgical planning.
Report Sentence
Fusiform expansion of the affected bone is observed with thinned but intact cortical bone; the lesion shows continuity with surrounding bone without distinct border.
On MRI, fibrous dysplasia shows variable signal intensity on T2-weighted sequences, and this signal characteristic reflects the histologic composition of the lesion. Lesions with predominant dense fibrous tissue component show low T2 signal — due to the short T2 relaxation time of collagen fibers. Areas with cystic degeneration or secondary aneurysmal bone cyst formation show high T2 signal. Woven bone matrix produces intermediate signal. This heterogeneous T2 signal pattern reflects the 'varied internal structure' of the lesion. After gadolinium, the fibrous component of the lesion shows enhancement — the degree of enhancement is proportional to the vascularity of fibrous tissue.
Report Sentence
The lesion shows heterogeneous signal intensity on T2-weighted sequences, with predominantly hypointense fibrous component and focal hyperintense cystic areas.
On T1-weighted sequences, fibrous dysplasia shows low-to-intermediate signal intensity. Fibrous stroma and woven bone matrix produce isointense or mildly hypointense signal compared to muscle. Normal bone marrow fat content is absent (normal fatty marrow is T1 hyperintense) because the medullary cavity is replaced by fibrous tissue — this helps differentiate from normal bone. On post-contrast T1 fat-sat sequences, the fibrous component shows moderate enhancement. The degree of enhancement is proportional to the vascularity and cellularity of fibrous stroma. Cystic degeneration areas show no enhancement (central non-enhancing) but cyst walls may enhance.
Report Sentence
The lesion shows low-to-intermediate signal intensity on T1-weighted sequences without normal bone marrow signal; moderate enhancement is present on post-contrast sequences.
On bone scintigraphy (Tc-99m MDP), fibrous dysplasia lesions show increased radiotracer uptake. The uptake is usually intense and homogeneous throughout the entire lesion. This increased uptake reflects the active bone metabolism and osteoblastic activity in fibrous dysplasia. Bone scintigraphy is the most sensitive modality for detecting the polyostotic form and evaluating the entire skeletal distribution of the disease — it plays a critical role in detecting other involved sites in cases thought to be monostotic. SPECT/CT anatomically localizes uptake foci. FDG uptake on PET-CT is variable and unreliable for differentiating from malignancy.
Report Sentence
Increased radiotracer uptake is observed in the __ region on bone scintigraphy, reflecting increased metabolic activity consistent with fibrous dysplasia; no additional/additional involved sites have been identified.
Cystic degeneration and secondary aneurysmal bone cyst formation may be seen in fibrous dysplasia lesions. On CT, low-density (0-20 HU) fluid-containing cystic areas are observed within the ground-glass density bone matrix. The cystic component is usually well-defined and surrounded by a thin sclerotic wall. Aneurysmal bone cyst complication may cause rapid expansion and local pain. Fluid-fluid levels may be observed within cystic areas — reflecting gravitational layering of hemorrhagic products of different ages. Cystic degeneration is a consequence of the chronic nature of fibrous dysplasia and is not an indicator of aggressive behavior or malignant transformation.
Report Sentence
Cystic degeneration areas and/or fluid-fluid levels are observed within the fibrous dysplasia lesion; secondary aneurysmal bone cyst complication should be considered.
Criteria
Involves a single bone. Constitutes 70-80% of all fibrous dysplasia cases. Single focus involvement should be confirmed on bone scintigraphy. Usually asymptomatic or mildly symptomatic. No McCune-Albright syndrome association.
Distinct Features
Isolated bone involvement, typical ground-glass density, fusiform expansion. In paranasal sinuses, maxilla is the most commonly involved bone. Growth tends to stop at puberty. Surgery is performed only for symptomatic (sinus obstruction, orbital compression) or cosmetic indications.
Criteria
Involves two or more bones. Constitutes 20-30% of all fibrous dysplasia cases. Usually unilateral or segmental distribution (sclerotome-based). Craniofacial involvement common. Entire skeleton should be screened with bone scintigraphy.
Distinct Features
Multiple bone involvement, earlier onset, potential for more aggressive course. 'Shepherd's crook' deformity (proximal femur). Increased risk of pathologic fracture. May be associated with McCune-Albright syndrome.
Criteria
Classic triad: polyostotic fibrous dysplasia + café-au-lait spots (irregular borders — 'coast of Maine') + endocrine disorders (precocious puberty most common, hypothyroidism, GH excess, Cushing). GNAS1 mutation. Multiple organ involvement.
Distinct Features
Polyostotic involvement, endocrine anomalies (precocious puberty, thyroid nodules, adrenal hyperplasia), café-au-lait spots (tendency to be ipsilateral to bone involvement), hepatic involvement. Ground-glass density and bone expansion in multiple bones on imaging. Endocrine screening is mandatory.
Distinguishing Feature
Osteoma shows very high bone density (>1000 HU in compact type) — distinctly different from fibrous dysplasia's ground-glass density (56-90 HU). Osteoma is a well-defined intraluminal mass while fibrous dysplasia shows diffuse involvement with bone expansion. Osteoma is distinctly separate from surrounding bone while fibrous dysplasia blends with surrounding bone.
Distinguishing Feature
Ossifying fibroma is a mixed-density lesion surrounded by a well-defined sclerotic capsule (shell sign) — fibrous dysplasia lacks capsule and distinct borders. Ossifying fibroma grows as an expansile mass while fibrous dysplasia shows bone expansion. Ossifying fibroma can be completely removed by surgical enucleation (due to capsule) while complete resection of fibrous dysplasia is difficult.
Distinguishing Feature
SCC demonstrates aggressive bone destruction — irregular osteolysis, cortical disruption, soft tissue mass. Fibrous dysplasia shows bone expansion, no destruction, cortical continuity preserved. SCC is soft tissue density (40-60 HU), fibrous dysplasia is ground-glass bone density (56-90 HU). Biopsy is needed when malignant transformation is suspected (rapid growth, pain, new soft tissue component).
Urgency
routineManagement
surveillanceBiopsy
Not NeededFollow-up
Asymptomatic with typical imaging: no routine follow-up needed, re-image if symptomatic. Growing lesions in children: annual CT until skeletal maturity. Polyostotic: bone scintigraphy for full skeletal survey, endocrine workup for McCune-Albright. Surgical: contouring/debulking for cosmetic or functional indications.Diagnosis is made with typical CT findings (ground-glass density + bone expansion) and biopsy is not needed in most cases. Asymptomatic monostotic lesions do not require treatment. Surgical indications: functional problems (sinus obstruction, orbital compression, optic canal narrowing), cosmetic deformity, pathologic fracture. Surgical contouring/debulking is performed — complete resection is usually not possible and recurrence is common (25-50%). Radiotherapy is contraindicated (increases malignant transformation risk). Bisphosphonate therapy may be tried for painful lesions. Bone scintigraphy and McCune-Albright screening (endocrine workup) should be performed in polyostotic form.
Fibrous dysplasia is a benign lesion that usually slows after puberty. Surgical contouring is performed for symptomatic cases (cosmetic deformity, sinus obstruction, visual impairment). Malignant transformation is very rare (0.5%), risk increases in irradiated areas.