Ground-glass matrix

• Fibrous dysplasia
• Non-ossifying fibroma
• Bone cyst

Non-aggressive lucent lesion in the proximal tibia. Differential includes fibrous dysplasia, non-ossifying fibroma, or bone cyst.

Final diagnosis: biopsy proven fibrous dysplasia

Board Tip: If polyostotic, consider McCune-Albright syndrome (FD + café-au-lait spots + endocrine dysfunction).

Biopsy proven osteoblastoma

Osteoblastoma

• Histologically similar to osteoid osteomas but are larger (>2 cm) and typically do not respond to NSAIDs.
• Common sites: Spine (posterior elements), long bones.
• Key imaging feature: Expansile, lytic lesion with variable matrix mineralization.
• Management: Surgical excision; radiofrequency ablation in select cases.

Nondisplaced transverse pathologic fracture through a lucent lesion in the distal third phalanx

Expansile appearance, no obvious chondroid calcifications in the lesion

Lucent Lesions of the Hand Bones

Enchondroma: Lytic, expansile, distal phalanx, ± pathologic fracture; no matrix in small bones

Giant Cell Tumor (GCT): Epiphyseal/metaphyseal, "soap-bubble" appearance, age 20–40, rare in phalanges

Aneurysmal Bone Cyst (ABC): Expansile "blowout" lesion, fluid-fluid levels on MRI, younger patients

Epidermoid Inclusion Cyst: History of penetrating trauma, sclerotic rim, soft tissue component

Expansile lytic phalangeal lesion with pathologic fracture, consistent with enchondroma.

Enchondroma (biopsy-proven)

Enchondroma

Key features: Benign cartilaginous tumor; 50% occur in hands/feet. In small bones, often purely lytic and expansile (vs. long bones, which show chondroid "rings and arcs" calcifications).

Mimics:

• GCT: Rare in phalanges, lacks expansile remodeling.
• ABC: More aggressive expansion, fluid-fluid levels.

Management: Observation if asymptomatic; curettage + bone grafting for pain/fracture.

Malignant transformation to chondrosarcoma is exceedingly rare in solitary hand lesions.

Expansile, lytic lesion in the radial shaft with cortical thinning. 

Fluid signal without septation on MRI.

There is also an osteochondral lesion in the lateral epicondyle with surrounding bone marrow edema.

1. Expansile lucent lesion in the proximal radius most compatible with a simple bone cyst.
2. Lateral epicondyle osteochondral lesion.

Cortical thinning might be introducing risk for pathological fracture.

Well-defined, lucent, cortically-based lesion in the posterior distal femur with a scalloped border and narrow zone of transition.

Lytic lesion in the upper outer quadrant of the patella.

Non-ossifying fibroma of the posterior distal femur (benign, incidental finding).

Dorsal defect of the patella (normal variant, no clinical concern).

Non-ossifying fibroma

• A frequently encountered lesion that should be recognized as benign and biopsy avoided
• Classic appearance: lucent lesions in the cortex of a long bone metaphysis with a well-defined, scalloped border with slight cortical expansion
• Commonly mistaken for: infection, eosinophilic granuloma, fibrous dysplasia, or aneurysmal bone cyst
• Usually asymptomatic

Dorsal defect of the patella

• A normal variant
• Lytic defect in the upper outer quadrant of the patella
• Can be mistaken for infection or osteochondritis dissecans
• A skeletal “don't touch” lesion

Non-aggressive lucent lesion in the distal femur, likely represents a non-ossifying fibroma.

Final diagnosis: biopsy proven non-ossifying fibroma

Usually a “don't touch” lesion. However, prophylactic curettage and bone grafting can be done to prevent a pathological fracture in large lesions (more than 50% of the bone diameter), which is what happened in this case.

Another example

Non-ossifying fibroma is a large fibrous cortical defect (both can be called a fibroxanthoma), with the difference being the size. >3 cm is the size distinction. 

They tend to get more mineralized over time. It is thought that bone islands are healed fibroxanthomas.

• Well-defined lucent lesion along the anterior aspect of the distal femur
• Diffuse loss of cortical definition and increased density of trabecular bone
• Subperiosteal resorption (best seen on the AP, proximal and medial tibia)
• Subligamentous bone resorption (see lateral and medial aspect of the femoral condyles)

• Probable Brown tumor in the distal femur related to hyperparathyroidism from end-stage renal disease.
• Diffusely increase bone density consistent with renal osteodystrophy.

Hyperparathyroidism can be primary as in a parathyroid adenoma, secondary as in chronic renal disease, or tertiary as in dialysis patients where the parathyroid gland may act independently of serum calcium levels.

Hyperparathyroidism stimulates osteoclastic resorption. This is responsible for the radiographic findings present in this case:

• Subperiosteal resorption
• Accentuated trabecular patterns
• Loss of cortical definition
• Subligamentous bone resorption
• Brown tumor (has been referred to as an osteoclastoma)

Brown tumors

• Accumulations of fibrous tissue with numerous osteoclastic giant cells
• Can mimic a destructive neoplasm
• Most common sites: mandible, pelvis, ribs, femora
• Can undergo hemorrhage with liquefactive necrosis and produce cysts
• Can show a fluid-fluid level on MRI

• Lytic lesion in distal radius
• Epiphyseal location with extension to the metaphysis
• Extension to articular surface
• Non-sclerotic, relatively narrow zone of transition seen proximally
• Minimal mineralized matrix
• Cortical thinning with pathologic fracture
• Prominent dorsal soft tissue swelling

Category: Epiphyseal Lytic Lesions

• Giant Cell Tumor: Eccentric, epiphyseal, extends to articular surface, no matrix, adults
• Chondroblastoma: Central epiphyseal, younger patients (10-25y), thin sclerotic rim, stippled calcifications
• Clear Cell Chondrosarcoma: Older patients, sclerotic rim, may have chondroid matrix
• Intraosseous Ganglion: Smaller, subchondral cyst-like lesion, no soft tissue component
• Metastasis: Important consideration for any lytic lesion in an older adult. 

Pathology: Giant Cell Tumor

Discussion:

• Key Points: Locally aggressive benign tumor composed of osteoclast-like giant cells, peak age 20-40 (unusual in this 65y patient), slight female predominance, metaphyseal-epiphyseal location in skeletally mature patients
• Mimics/Pitfalls: Can be confused with other epiphyseal lesions; requires correlation with age and exact location
• Management: Curettage with cementing/bone grafting for smaller lesions; en bloc resection for aggressive/recurrent cases; denosumab for unresectable tumors
  
   

Key takeaway: When you see a lytic epiphyseal lesion extending to an articular surface, think giant cell tumor.

Lucent lesion in the medial femoral condyle, extending to the subarticular bone, with a mildly displaced pathologic fracture. Imaging features are highly suggestive of a giant cell tumor (GCT). Histologic confirmation is recommended.

Giant Cell Tumor

Biopsy proven

Key Points:

1. Classic location → Most (50-60%) occur around the knee (distal femur, proximal tibia).
2. Epiphyseal, extends to subchondral bone
3.  Always occurs after physeal closure.
4. Purely lytic, no matrix mineralization → Helps differentiate from chondroid or osteoid-producing lesions.
5. Cortical thinning and breakthrough → Can be locally aggressive but lacks a true periosteal reaction.
6. Narrow zone of transition without a sclerotic rim → Unlike non-ossifying fibroma or other benign lytic lesions.

Board tip: GCT occurs in skeletally mature patients, abuts the articular surface, and is purely lytic without a sclerotic rim. 

• Multiple lytic lesions in the left proximal femur.
• No pathologic fracture identified.

Metastatic lesions in the proximal femur.

This patient had metastatic carcinoma of unknown primary.

Aneurysmal bone cyst with pathologic fracture.