Osteochondroma (exostosis) is the most common benign bone tumor, growing as a cartilage-capped bony projection from the bone surface. Chest wall osteochondromas most frequently arise from the ribs and scapula, and rarely from the sternum and clavicle. They are typically detected during childhood and adolescence, with growth ceasing after skeletal maturity. The pathological hallmark of the lesion is cortical and medullary continuity with the underlying parent bone (pathognomonic finding). The cartilage cap in benign lesions is generally <2 cm thick; thickness >2 cm should raise suspicion for malignant transformation (secondary chondrosarcoma). Solitary osteochondromas are the most common form; multiple hereditary exostoses (MHE/HME) is an autosomal dominant condition with multiple osteochondromas. The risk of malignant transformation is 1-2% for solitary forms and 5-10% for MHE. Symptoms are usually related to soft tissue compression, bursitis, or mechanical irritation — scapular osteochondromas in particular can cause scapulothoracic crepitus. Complications from great vessel or pleural compression in chest wall osteochondromas are rare but reported, making preoperative vascular assessment important.
Age Range
10-40
Peak Age
20
Gender
Male predominant
Prevalence
Uncommon
Osteochondroma develops as a result of aberrant cartilage proliferation arising from the growth plate. During normal growth plate activity, a defect in the perichondral ring (Ring of Ranvier) causes herniation of physeal cartilage cells toward the bone surface, creating an ectopic cartilage island. This ectopic cartilage converts to bone through endochondral ossification, producing the osteochondromatous projection. Histologically, the tip of the lesion bears a hyaline cartilage cap; beneath this cap, the medullary cavity and cortical bone are in direct continuity with the parent bone. Cortical and medullary continuity confirms the lesion is a growth plate anomaly rather than a true neoplasm and is the pathognomonic finding on imaging: on CT, the cortical bone line and medullary fat signal extend uninterrupted into the lesion. The cartilage cap thickness reflects cartilage proliferative activity — it may be thick (1-3 cm) in children during growth, and thins (<0.5-1 cm) after skeletal maturity. Cap thickness >2 cm in adults should raise concern for malignant transformation (secondary chondrosarcoma); at this point, cartilage cells show uncontrolled proliferation, with atypical appearances including disrupted medullary continuity and irregular mineralization in the cap. On MRI, the cartilage cap is T2 hyperintense because hyaline cartilage has high water content — long T2 relaxation of water molecules produces hyperintense signal. On CT, the cartilage cap appears low density because unmineralized cartilage matrix is significantly less dense than bone; however, punctate calcifications may be seen within the cap, representing early stages of endochondral ossification.
Uninterrupted continuation of the cortical bone layer and medullary cavity from the parent bone into the lesion — the pathognomonic finding distinguishing osteochondroma from other bone lesions. Best evaluated on multiplanar reformations (coronal, sagittal).
CT demonstrates uninterrupted continuation of the cortical bone layer and medullary cavity from the parent bone into the lesion — this is the pathognomonic finding. Sessile lesions show continuity over a broad base, while pedunculated forms show continuity through a narrow stalk. Coronal and sagittal reformations are essential for this continuity assessment; axial sections alone can be misleading. Normal fat-density marrow (-50 to -100 HU) within the medullary cavity confirms the benign nature of the lesion. Disruption of cortical continuity or solid soft tissue density within the medullary cavity should raise suspicion for malignant transformation.
Report Sentence
The bony projection arising from the rib/scapula demonstrates cortical and medullary continuity, a pathognomonic finding consistent with osteochondroma (exostosis).
Osteochondromas present in two main morphological types: sessile (broad-based, slowly rising from the bone surface) and pedunculated (extending from the parent bone on a narrow stalk). Both types are seen in chest wall osteochondromas; costal osteochondromas are more commonly sessile while scapular lesions may be pedunculated. In pedunculated lesions, the stalk points away from the growth plate — this is a helpful diagnostic clue confirming growth plate origin. Corticomedullary continuity assessment is easier in sessile lesions due to the broad base, while the narrow stalk of pedunculated lesions may cause partial volume artifact. Both types show a thin periosteal layer and perichondrium on the surface.
Report Sentence
The lesion demonstrates sessile/pedunculated morphology with corticomedullary continuity with the parent bone.
On T2-weighted MRI sequences, the cartilage cap at the tip of the lesion shows markedly hyperintense signal — reflecting the high water content of hyaline cartilage. Cap thickness is critically important diagnostically: <2 cm thickness is consistent with a benign lesion, while >2 cm thickness should raise suspicion for malignant transformation (secondary chondrosarcoma). T2 signal heterogeneity and nodular thickening within the cap are also suspicious findings. Cap measurement should always be taken at the thickest point. The cap is similarly hyperintense on STIR; on T1, the cap is isointense or slightly hypointense relative to muscle.
Report Sentence
A hyperintense cartilage cap is seen at the tip of the lesion on T2-weighted sequences, measuring ___ mm in thickness; <2 cm thickness is consistent with benign osteochondroma.
On T1-weighted MRI sequences, the medullary cavity of the lesion contains the same fat signal (T1 hyperintense) as the parent bone marrow with no interruption between them. This finding is the MRI correlate of the corticomedullary continuity principle and confirms benign osteochondromatous structure. T1 hypointense solid tissue within the medullary cavity — especially combined with >2 cm cartilage cap — should raise concern for malignant transformation. Absence of enhancement within the medullary cavity on post-contrast fat-suppressed T1 sequences further supports benign nature; enhancement in the cap or medullary cavity is atypical.
Report Sentence
On T1-weighted sequences, the medullary cavity of the lesion shows fat signal continuity with the parent bone marrow.
Punctate or arcuate calcifications may be seen within the cartilage cap — these represent early stages of endochondral ossification. In benign lesions, calcifications typically concentrate at the cap-bone interface and are well-defined. In malignant transformation, calcification pattern becomes irregular: scattered, clustered, or amorphous calcifications are seen, accompanied by asymmetric cap thickening. Ring-and-arc calcification pattern represents chondroid matrix mineralization and confirms the chondroid nature of osteochondroma. In the chest wall, calcifications in costal osteochondromas can be confused with costal cartilage calcifications — multiplanar reformations help differentiate.
Report Sentence
Punctate calcifications are seen within the cartilage cap, reflecting chondroid matrix mineralization (ring-and-arc pattern).
On post-contrast MRI, the cartilage cap of a benign osteochondroma typically shows no enhancement or minimal perichondrial enhancement — when active cartilage metabolism is limited, there is no neovascularization. However, in growing children, mild cap enhancement may be a normal finding due to active endochondral ossification. In adults, prominent, heterogeneous, or nodular enhancement in the cap is a suspicious finding for malignant transformation (secondary chondrosarcoma). The presence of an enhancing mass in perilesional soft tissue also indicates aggressive behavior. Post-contrast fat-suppressed T1 sequences are the preferred sequences for enhancement assessment.
Report Sentence
No significant enhancement is observed in the cartilage cap on post-contrast fat-suppressed sequences; consistent with benign osteochondroma.
Ultrasonography can be used to assess the cartilage cap in superficially located chest wall osteochondromas. The cartilage cap appears as a hypoechoic, homogeneous layer over the bone surface. Cap thickness can be measured by US — this measurement correlates with MRI. The bone surface creates strong acoustic shadowing, preventing assessment of deeper parts of the lesion. US is a non-invasive and reproducible method for cartilage cap thickness follow-up; however, MRI is preferred for deeply located lesions.
Report Sentence
A hypoechoic cartilage cap is seen overlying the bony projection on US, measuring ___ mm in thickness.
Criteria
Grows outward from bone surface on a broad base; more common in costal osteochondromas
Distinct Features
Corticomedullary continuity assessment easy; broad base visible on multiple CT sections; lower malignant transformation risk than pedunculated
Criteria
Grows away from parent bone on a narrow stalk; stalk points away from growth plate
Distinct Features
More mobile, higher risk of mechanical compression on surrounding tissues; more common in scapular osteochondromas; narrow stalk may cause partial volume artifact — MPR essential
Criteria
Autosomal dominant inheritance (EXT1/EXT2 gene mutation); multiple osteochondromas; childhood diagnosis
Distinct Features
5-10% malignant transformation risk (higher than solitary); multiple chest wall lesions possible; short stature and joint deformities may be associated; screening MRI protocol recommended
Criteria
Cartilage cap >2 cm; size increase in adulthood; development of pain; irregular cap enhancement
Distinct Features
Cap thickness >2 cm (pathognomonic suspicion threshold); disrupted cortical continuity; perilesional soft tissue mass; nodular irregular calcification in cap; increased FDG uptake on PET-CT
Distinguishing Feature
Chondrosarcoma: Cortical destruction, periosteal reaction, soft tissue mass; cartilage cap >2 cm with irregular enhancement. Osteochondroma: Corticomedullary continuity preserved, cap <2 cm, no/minimal enhancement
Distinguishing Feature
Enchondroma: Intramedullary location, intraosseous lesion; typical ring-and-arc calcification; no exophytic projection. Osteochondroma: Exophytic projection from bone surface; corticomedullary continuity; external cartilage cap
Distinguishing Feature
Osteosarcoma: Aggressive bone destruction, periosteal reaction (sunburst, Codman triangle), osteoid matrix, soft tissue mass. Osteochondroma: Corticomedullary continuity preserved, no aggressive features, cartilage cap present
Distinguishing Feature
Fibrous dysplasia: Intramedullary, ground-glass matrix, bone expansion; no exophytic projection. Osteochondroma: Exophytic bony projection, normal medullary fat signal, cartilage cap
Urgency
routineManagement
surveillanceBiopsy
Not NeededFollow-up
12-monthAsymptomatic osteochondromas with typical imaging findings are generally managed with conservative follow-up. If cartilage cap thickness is <2 cm, corticomedullary continuity is preserved, and growth has ceased, biopsy is not needed. Symptomatic lesions (pain, mechanical limitation, scapulothoracic crepitus) or complications (vascular/nerve compression, bursitis) are indications for surgical resection. In cases of suspected malignant transformation (cap >2 cm, growth, pain), urgent further evaluation (MRI, biopsy) is required. Regular screening MRI is recommended for MHE patients.
Osteochondromas generally require no treatment and are followed with surveillance. Surgical excision is performed for symptomatic lesions (nerve compression, cosmetic concern). Cartilage cap >2 cm or increasing growth rate should be evaluated for malignant transformation (secondary chondrosarcoma, 1-2% risk). Multiple hereditary exostoses (MHE/HME) carry a higher risk of malignant transformation (5-10%).