Fibrous dysplasia is a benign fibro-osseous lesion in which normal bone is replaced by fibrous tissue and immature bone trabeculae. In the chest wall, it most commonly involves the ribs and is usually discovered incidentally. It is characterized by pathognomonic 'ground-glass' matrix density on CT, well-defined expansile remodeling, and sclerotic rim. It presents in monostotic (70-80%) or polyostotic (20-30%) form, and the polyostotic form may be associated with McCune-Albright syndrome (cafe-au-lait spots + precocious puberty + polyostotic fibrous dysplasia). The risk of malignant transformation is very low (<1%) but radiation history increases the risk.
Age Range
10-40
Peak Age
25
Gender
Equal
Prevalence
Uncommon
Fibrous dysplasia results from a postzygotic activating somatic mutation in the GNAS1 gene (20q13.3). This mutation leads to constitutive activation of the Gsα subunit, increasing cAMP production and impairing osteoblast differentiation. Consequently, normal lamellar bone is replaced by disorganized, immature woven bone trabeculae (Chinese letter pattern) within a fibrous stroma. These trabeculae lack an osteoblastic rim (aplastic bone). The ground-glass appearance of the lesion reflects the CT attenuation of these irregularly mineralized trabeculae within the fibrous stroma — neither fully bone nor fully soft tissue density. In monostotic form, the mutation occurs in a limited number of cells during late embryonic development, while in polyostotic form it occurs earlier and in more widespread cells (mosaicism).
This is the pathognomonic CT finding of fibrous dysplasia. A homogeneous, intermediate attenuation (70-130 HU) matrix density within the lesion lies between normal bone and soft tissue. This appearance is created by irregularly mineralized immature woven bone trabeculae within the fibrous stroma. The ground-glass density is so characteristic that when combined with typical expansile remodeling and smooth contour, it can establish the diagnosis alone and biopsy is often unnecessary. CT demonstrates this matrix characteristic much more reliably than MRI.
Homogeneous ground-glass matrix density within an expansile rib lesion. Attenuation values are between normal bone and soft tissue (typically 70-130 HU). This intermediate density is created by irregularly mineralized immature bone trabeculae within the fibrous stroma. The matrix is generally homogeneous but focal cystic areas, cartilaginous nodules, or calcifications may create heterogeneity.
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A well-defined expansile lesion measuring approximately _x_ cm with homogeneous ground-glass matrix density (HU: ~___) is observed in the [Xth] rib; compatible with fibrous dysplasia.
The lesion expansively widens the rib contour but bone remodeling is orderly and symmetric. A thin sclerotic rim (cortical shell) is usually intact — focal cortical breach is rare and generally associated with pathologic fracture. Endosteal scalloping may be observed. There is NO periosteal reaction (aggressive pattern is not expected).
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The lesion shows orderly expansile remodeling with intact thin sclerotic cortical shell; no periosteal reaction is observed.
Fibrous dysplasia generally shows low to intermediate signal intensity on T1-weighted sequences. Signal may be iso- or hypointense relative to muscle. This low T1 signal reflects the fibrous stroma content. Focal areas of high T1 signal may suggest hemorrhage or cystic degeneration. The normal high T1 signal of fatty bone marrow is lost.
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The lesion shows low to intermediate signal intensity on T1-weighted sequence with loss of normal bone marrow fat signal.
T2 signal intensity is highly variable and depends on the histological composition of the lesion. Fibrous tissue-dominant lesions show LOW T2 signal (collagen-dependent short T2). Areas of cystic degeneration or myxoid change show HIGH T2 signal. Cartilaginous nodules may also show high T2 signal. This variability is the diagnostic pitfall of fibrous dysplasia on MRI — low T2 signal may mimic malignancy, high T2 signal may mimic cystic lesions.
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The lesion shows variable signal intensity on T2-weighted sequences, predominantly low/intermediate/high signal [depending on fibrous/cystic/mixed component presence].
Enhancement pattern is variable. Fibrous tissue-dominant lesions show minimal to moderate enhancement. Vascular fibrous stroma areas may demonstrate prominent enhancement. Cystic areas show no enhancement. Generally, a progressive enhancement pattern compatible with benign lesion is expected (rapid wash-in + washout favors malignancy).
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The lesion shows variable enhancement on post-contrast series, with minimal/moderate/prominent enhancement in solid components while cystic areas show no enhancement.
Well-defined expansile lesion of the affected rib. Ground-glass matrix density — less opaque than normal bone but more opaque than soft tissue. Thin sclerotic rim may be present. 'Shepherd's crook' deformity is typical in long bones; in the rib, expansile fusiform widening is observed. No periosteal reaction.
Report Sentence
A well-defined expansile lesion with ground-glass matrix density is observed in the [Xth] rib, compatible with fibrous dysplasia.
Fibrous dysplasia lesions show increased radiopharmaceutical uptake on 99mTc-MDP bone scintigraphy. Uptake can be prominent and may mimic malignant lesions. In polyostotic form, all involved bones can be detected in a single study. In monostotic form, a solitary focus of increased uptake is observed.
Report Sentence
Increased radiopharmaceutical uptake is observed at the level of the [Xth] rib; assessed as related to the known fibrous dysplasia lesion. No additional/Additional uptake foci are detected in other bones.
Criteria
Most common form (70-80%). Single bone involved. Rib, femur, tibia, craniofacial bones are most common locations. Usually discovered incidentally at age 20-30 years. No endocrine anomaly association. Malignant transformation risk is very low (<0.5%). Growth usually ceases after puberty.
Distinct Features
Single bone involvement, no endocrine anomaly, stabilization after puberty
Criteria
More than one bone involved (20-30%). Usually unilateral distribution. Symptomatic earlier (childhood). Higher risk of pathologic fracture. Cafe-au-lait spots may be associated. Malignant transformation risk higher than monostotic form (<4%).
Distinct Features
Multiple bone involvement, unilateral distribution, cafe-au-lait spots, early age onset
Criteria
Polyostotic fibrous dysplasia + cafe-au-lait spots (irregular 'coast of Maine' borders) + precocious puberty (or other endocrine anomalies — hyperthyroidism, growth hormone excess, Cushing). Results from extensive mosaicism of GNAS1 mutation. Course includes severe skeletal deformities, pathologic fractures, and endocrine complications.
Distinct Features
Polyostotic involvement + cafe-au-lait (coast of Maine border) + endocrine anomaly triad
Criteria
Fibrous dysplasia (usually polyostotic) + intramuscular myxomas. A rare association. Myxomas generally develop in muscles adjacent to bones affected by fibrous dysplasia. Seen on MRI as T2 hyperintense, well-defined soft tissue mass. Low malignancy risk.
Distinct Features
Fibrous dysplasia + intramuscular myxoma in adjacent muscle, T2 hyperintense soft tissue lesion
Distinguishing Feature
Expansile lytic lesion but fluid density on CT (0-20 HU) — NOT ground-glass density. Low T1, high T2 signal on MRI (fluid). Thin sclerotic rim and cortical thinning may be similar. No enhancement (except wall enhancement). Usually at younger age.
Distinguishing Feature
Cartilaginous matrix mineralization — 'popcorn' or 'arc-and-ring' calcification on CT (different from ground-glass density of fibrous dysplasia). T2 hyperintense lobular mass on MRI (high water content of cartilage). Rib enchondromas are not uncommon but usually localized to the anterior rib end. Peripheral and septal enhancement pattern.
Distinguishing Feature
Fat density on CT (-40 to -100 HU) — very different from ground-glass density. High T1 signal on MRI (fat), signal loss on fat-suppressed sequences. Well-defined, may be expansile. May contain central calcification or cystic degeneration areas. Minimal enhancement.
Distinguishing Feature
The malignant tumor most resembling fibrous dysplasia — may show ground-glass-like matrix but cortex destruction and soft tissue component are not expected in fibrous dysplasia. Periosteal reaction, cortical breach, and surrounding soft tissue invasion favor malignancy. Growth or character change during fibrous dysplasia follow-up suggests malignant transformation. Biopsy may be necessary for differential diagnosis.
Urgency
lowManagement
Fibrous dysplasia generally does not require treatment — observation is sufficient for asymptomatic lesions. In symptomatic cases (pain, pathologic fracture, cosmetic deformity), surgical curettage or resection may be performed but recurrence rate is high (25-50%). Bisphosphonates (especially pamidronate, zoledronic acid) have been found effective for pain control and lesion stabilization. Standard fracture treatment applies for pathologic fractures. Radiotherapy is CONTRAINDICATED — increases the risk of malignant transformation. Treatment of endocrine anomalies is necessary in McCune-Albright syndrome.Biopsy
Not NeededFollow-up
Routine follow-up is not required for asymptomatic monostotic lesion with typical ground-glass density. In polyostotic form, complete mapping with bone scintigraphy and endocrine evaluation are required. Rapid growth, increased pain, or change in imaging character (new soft tissue component, cortical destruction) is concerning for malignant transformation — requires biopsy. Periodic follow-up (annual imaging) may be considered for lesions at risk of pathologic fracture. Increased risk of malignant transformation in patients with radiation history — closer follow-up needed.Fibrous dysplasia is a benign fibro-osseous lesion and diagnosis is usually certain with typical ground-glass density on CT — biopsy is NOT NEEDED in most cases. Treatment or follow-up is not mandatory for asymptomatic monostotic rib lesions. However, a multidisciplinary approach (orthopedics, endocrinology) is necessary in the presence of polyostotic form and McCune-Albright syndrome. The most important clinical question is exclusion of malignant transformation — biopsy is indicated when growth, pain, or character change occurs.
Rib fibrous dysplasia is generally benign and asymptomatic, requiring no treatment. Large expansile lesions carry pathologic fracture risk. Malignant transformation is extremely rare (<0.5%). In polyostotic forms, McCune-Albright syndrome should be investigated (cafe-au-lait spots, precocious puberty, hyperthyroidism). In suspicious cases (aggressive features, growth), biopsy should be considered to exclude chondrosarcoma. Follow-up CT confirms lesion stability.