Epidemiology
These are benign bone-forming lesions and they are very common, almost always turning up as an incidental finding on a radiograph or CT requested for another reason. An osteoma grows on the surface of the bone; an enostosis —also called a bone island— is a focus of compact bone inside the marrow, and is seen frequently in the pelvis, the femur and the vertebrae. They can be found at any age and are not associated with any increased risk of cancer.
Basic features
On imaging they appear as a focus of dense, homogeneous bone with well-defined margins, which does not destroy the cortex or lift the periosteum. They do not usually hurt: when there is pain in the area, it almost always has another cause worth looking for. They do not grow, or they grow very slowly. The reason a report mentions them is that on a radiograph a bone island can resemble a sclerotic metastasis (a dense lesion from another cancer); what distinguishes it is that a bone island has spiculated margins that blend into the neighboring trabecular bone, stays stable over time, and causes no symptoms.
Usual treatment
An osteoma or an enostosis with a typical appearance needs no treatment, no biopsy and no follow-up: knowing what it is, is enough. Further study is only warranted when there is reasonable doubt —for example, if the person has a known cancer and it has to be distinguished from a sclerotic metastasis, or if the lesion grows or is associated with persistent pain— and in those cases a CT, an MRI or a bone scan is used, and occasionally a follow-up radiograph to confirm that it does not change. Resection is considered only exceptionally, when a surface osteoma is bothersome because of its location.
Outcomes and prognosis
The prognosis is excellent: these are benign lesions, they do not turn into cancer and they do not limit activity. In most cases the value of identifying them lies precisely in being able to stop worrying about them and to avoid unnecessary tests.