Skeletal benign bone-forming lesions.

The imaging features of benign osseous lesions of the bone are often characteristic and suggestive of a specific diagnosis. This is particularly true for skeletal benign bone-forming lesions such as enostosis, osteoma, osteoid osteoma and osteoblastoma. Enostosis or bone island is an incidental finding in the axial skeleton (pelvis, spine, ribs) of asymptomatic patients; it appears as a small (0.2-2 cm) round to oval sclerotic area with irregular, radiating ('thorny') spicules peripherally. Osteoma is a benign slow-growth tumor and usually an incidental finding in cranial sinuses, vault and mandible, and presenting as a homogeneous, sharply defined bone mass arising from bone surface; its signs and symptoms are rare and depend on the tumor size and location--e.g. sinusitis, headache, exophthalmos, diplopia. Osteoid osteoma is a painful highly vascularized benign tumor usually affecting the long bone diaphysis cortex of young patients; it generally appears as a small radiolucent nidus with or without central calcification and surrounding bone sclerosis on radiographs, and as a 'hot' spot on scintigraphy. CT is the method of choice for the definite location of the nidus especially in sites of complex anatomy, such as the spine, pelvis and hindfoot. Osteoblastoma is a rare tumor, histologically similar to osteoid osteoma but with a significantly different clinical potential because of the possibilities of postoperative recurrence, of its locally aggressive behavior or, rarer still, malignant transformation; the spine and long bones are affected in more than half the cases. Its radiologic appearance is not always distinctive and usually characterized by a lytic lesion with varying bone production and expansile behavior; CT and MRI are required for the diagnosis of spinal osteoblastomas. When a bone-producing tumor or tumor-like lesion is suspected but no specific diagnosis can be made, the knowledge of the range of the imaging findings of these lesions will allow a suitably ordered differential diagnosis. Radiography is the single most effective imaging method in this respect. CT is required for the tumors in complex anatomical sites, such as the spine, pelvis and hindfoot, as well as for the optimal assessment of the tumor matrix. MRI is specifically required to study the lesion effect on the spinal canal.