What does a densely sclerotic osseous (bone) lesion in the anterior aspect of the calcaneus (heel bone) represent?

Densely Sclerotic Calcaneal Lesion: Bone Island (Enostosis)

A densely sclerotic 18 mm osseous lesion in the anterior calcaneus with characteristic radiographic features most likely represents a bone island (enostosis), a benign developmental focus of compact bone within cancellous bone that requires no treatment or follow-up if imaging features are典型. 1

Diagnostic Features of Bone Islands

Radiographic Characteristics

• Homogeneously dense, sclerotic focus within cancellous bone with distinctive radiating bony streaks ("thorny radiation" or "pseudopodia") that blend with surrounding trabeculae, creating a feathered or brush-like border 2, 1
• The 18 mm size falls within the range of typical bone islands (most measure 0.1-2.0 cm, though giant bone islands >2 cm are well-documented) 3, 1
• Ovoid, round, or oblong shape with well-defined margins 1

Key Imaging Pearls

• CT demonstrates low-attenuation focus with the characteristic thorny radiations extending into adjacent trabeculae 1
• MRI shows low signal intensity on all sequences, similar to cortical bone 1
• The ACR notes that dense sclerotic bone lesions without signal on diffusion-weighted images should not have ADC measurements performed, as these are unreliable in such lesions 4

Critical Diagnostic Pitfall: Bone Scan Activity

The most important caveat is that bone islands can show increased radiotracer uptake on bone scintigraphy, which does NOT indicate malignancy. 2, 5

• Histologically confirmed bone islands may be "hot" on bone scan due to increased osteoblastic activity within the lesion 2, 5
• The morphologic features on radiographs and CT are the guide to correct diagnosis, NOT the degree of scintigraphic activity 2, 5
• An asymptomatic, isolated sclerotic lesion with feathered borders is most likely an enostosis regardless of bone scan activity 2

Diagnostic Algorithm for Sclerotic Calcaneal Lesions

When Bone Island is Confirmed

• If radiographic features show characteristic thorny radiations and brush-like borders, no further imaging or biopsy is needed 2, 1
• No follow-up imaging is required for lesions with典型 features 2
• Bone islands are typically asymptomatic incidental findings 1

When Features Are Atypical

According to ACR Appropriateness Criteria, if the lesion lacks characteristic features or appears aggressive 4:

• Obtain radiographs first if not already performed, as they remain the most appropriate initial imaging modality for bone lesions 4
• CT without contrast can better characterize matrix mineralization and cortical integrity 4
• MRI with contrast may be indicated if malignancy cannot be excluded, particularly to assess soft tissue extension 4

Growth Potential and Long-Term Behavior

• While traditionally considered stable, documented cases show bone islands can enlarge over decades (one case showed marked growth over 31 years) 6
• Stability or slow growth with典型 radiographic features and normal bone scan strongly favors benign bone island over malignancy 6
• The presence of growth does NOT mandate biopsy if morphologic features remain典型 6

When to Consider Alternative Diagnoses

The differential diagnosis for sclerotic calcaneal lesions includes 4:

• Osteoblastic metastasis (lacks thorny radiations, often multiple lesions, patient with known primary malignancy)
• Chronic osteomyelitis (clinical history of infection, periosteal reaction, soft tissue changes)
• Stress fracture (history of increased activity, tenderness on lateral calcaneal compression, may show on bone scan before radiographs) 4
• Fibrous dysplasia (ground-glass matrix, expansile, different age presentation)

Management Recommendation

For an 18 mm densely sclerotic anterior calcaneal lesion with characteristic thorny radiations and brush-like borders: no treatment, no biopsy, and no follow-up imaging is required. 2, 1 If the patient is symptomatic or radiographic features are atypical, proceed with CT for better characterization before considering biopsy 4.