What does a possible small bone island versus a fibrous lesion in the left posterior calvaria (calvaria refers to the upper part of the skull) mean for a 44-year-old female?

Understanding a Small Bone Island in the Posterior Calvaria

A small bone island in the posterior calvaria is typically a benign, non-progressive lesion that rarely requires treatment or intervention.

What Is a Bone Island?

A bone island (enostosis) is a focus of mature compact (cortical) bone within cancellous bone. These are benign lesions with the following characteristics:

• Typically asymptomatic and incidental findings on imaging
• Usually small (0.1-2.0 cm in diameter)
• Appear as homogeneously dense, sclerotic foci on imaging
• Often have characteristic radiating bony streaks ("thorny radiations")
• Most commonly found in the pelvis, femur, and other long bones, but can occur anywhere in the skeleton including the skull 1

Differentiating Bone Islands from Fibrous Lesions

When CT imaging suggests "possible small bone island versus fibrous lesion," the following distinctions are important:

Bone Island Features:

• Homogeneously dense, sclerotic appearance
• Well-defined borders that blend with surrounding bone
• Usually "cold" on bone scans (non-active)
• Low signal intensity on all MRI sequences (similar to cortical bone) 1

Fibrous Lesion Features:

• May include fibrous dysplasia, which shows a "ground glass" appearance on CT
• Often has more heterogeneous density
• May show enhancement with contrast
• Can be associated with expansion of bone 2

Clinical Significance

For a 44-year-old female with an incidentally discovered small bone island in the posterior calvaria:

1. Prognosis: Excellent - bone islands are benign and typically non-progressive
2. Symptoms: Usually asymptomatic, though rare cases of symptomatic bone islands have been reported 3
3. Follow-up: No specific follow-up is typically required for small, incidental bone islands
4. Intervention: Not typically necessary unless the lesion is symptomatic or shows atypical features

When to Consider Further Evaluation

Consider additional imaging or evaluation if:

• The lesion is larger than 2 cm (considered a "giant" bone island) 4
• There is associated pain that cannot be explained by other causes
• The lesion shows growth on follow-up imaging
• The lesion has atypical radiographic features

Imaging Modalities for Further Evaluation

If further evaluation is needed:

• MRI: Can help differentiate bone islands from other lesions based on signal characteristics 5
• CT with thin slices: Provides detailed assessment of bone architecture and can help confirm the diagnosis 5
• Bone scan: Traditionally used to differentiate bone islands (typically "cold") from more aggressive lesions, though some bone islands can show activity 1

Key Takeaways

1. Small bone islands in the calvaria are benign findings that rarely require intervention
2. They are typically stable over time and do not progress
3. No specific treatment or follow-up is usually necessary
4. Patients can be reassured about the benign nature of this finding

This incidental finding should not be a cause for concern, and the patient can be reassured that it is a common, benign variant of normal bone structure.