Bone island: scintigraphic findings and their clinical application.

PURPOSE: To evaluate bone islands that showed increased uptake of radiotracer on skeletal scintigraphy and to present an algorithm for examining such lesions to avoid misdiagnosis in difficult cases.

PATIENTS AND METHODS: Over an 8-year period, 20 patients (10 men and 10 women ranging in age from 33 to 82 years) with bone islands that showed activity on skeletal scintigraphy were examined with plain radiography (all patients), computed tomography (CT; 5 patients) and magnetic resonance imaging (MRI; 4 patients). For six of the patients the clinical presentation and the radiologic studies suggested malignancy, which prompted biopsy and histopathologic examination. Histopathologic study was also performed for six other patients in whom the bone islands were found incidentally during evaluation for joint replacement surgery for osteoarthritis. In the last eight patients the lesions exhibited the characteristic radiologic features of enostosis, and these patients were followed for up to 3 years without biopsy.

RESULTS: In all cases plain radiography showed the characteristic features of a bone island: a homogeneously dense, sclerotic focus in the cancellous bone with distinctive radiating bony streaks ("thorny radiation") that blended with the trabeculae of the host bone to create a feathered or brush-like border. Histopathologic examination of scintigraphically active bone islands showed increased osteoblastic activity, and the lesions were marked by a mixture of compact and trabecular bone. In the patients who did not undergo biopsy but were followed with radiologic examinations, there was no change in the size or appearance of the lesions.

CONCLUSIONS: The key to the correct diagnosis of bone island lies in the distinctive radiographic features of enostosis. An asymptomatic, isolated sclerotic bone lesion showing feathered or brush borders is most likely an enostosis, regardless of its size or its activity on scintigraphy. Therefore, a practical algorithm for examining bone islands should flow from their morphologic features as observed on radiographs and CT and MRI scans, rather than from their activity on scintigraphy.