Findings: Plain radiographs as well as CT images of the patient's right hip demonstrate a well-circumscribed lytic lesion within the anterior, subcapital region of the proximal right femur. The lesion has a central area of calcification as well as surrounding sclerosis.

Diagnosis: Osteoid osteoma.

Discussion: An osteoid osteoma is a benign osteoblastic tumor comprised of a central core of vascular osteoid tissue and a peripheral zone of sclerotic bone. Most patients with osteoid osteomas present with pain which is classically more severe at night and relieved with small doses of aspirin or NSAID's. Osteoid osteomas have been reported in all age groups; however, they most frequently occur in patients between the ages of seven and 25 years. The male to female ratio of occurrence is approximately three to one.

Osteoid osteomas occur more commonly in the long tubular bones, especially those in the lower extremities. Approximately 50 to 60% of osteoid osteomas occur in the femur or tibia. Within the femur, lesions are usually found proximally, especially within the neck and intertrochanteric region. When they occur in the spine, osteoid osteomas usually are seen in the posterior elements. Plain radiographs are often diagnostic with the classic appearance consisting of an oval or round radiolucent area measuring less than 1 cm in diameter surrounded by a zone of uniform osseous sclerosis. Within the long tubular bones, osteoid osteomas are usually diaphyseal in location and cortically based. The nidus of the lesion may be uniformly radiolucent or may contain variable amounts of calcification. In the carpal or tarsal bones as well as within the epiphyses of the long tubular bones, osteoid osteomas usually arise within the medullary spongiosa and appear radiographically as well-circumscribed lesions that are partially or completely calcified. In these regions, extensive reactive sclerosis is generally absent, and in fact, a radiolucent zone may surround the lesion. In the metacarpals, metatarsals, and phalanges, osteoid osteomas can have variable radiographic appearances.

Osteoid osteomas which occur in intraarticular sites often present with one or more of the following clinical findings: pain, soft tissue swelling, joint effusion, or decreased range of motion. These symptoms may be falsely attributed to a primary articular disorder, resulting in a delay in diagnosis. A delay in therapy, particularly within the hip, can result in secondary osteoarthritic changes including joint space narrowing, subchondral sclerosis, and osteophyte formation. A spinal osteoid osteoma is one of the causes of painful scoliosis, and excision of the lesion in adolescents usually results in complete resolution of the abnormal spinal curvature. Lesions are usually located along the concave aspect of the scoliotic curve, near its apex.

Plain radiographs are not always diagnostic of osteoid osteoma, particularly within the spine. Sometimes only osteosclerosis can be seen on plain radiographs, and the central radiolucent nidus cannot be visualized. In these instances, CT is usually performed and can often demonstrate the central radiolucent nidus which may or may not contain central calcifications. Osteoid osteomas demonstrate increased radiopharmaceutical activity on bone scintigraphy. MR imaging is usually not utilized for osteoid osteoma as MRI findings may simulate those of a malignant tumor or osteomyelitis due to the presence of marrow and soft tissue edema and even a soft tissue mass consisting of reactive, myxomatous tissue which may enhance following Gadolinium administration.

Classically, osteoid osteomas have been treated with surgical resection. Resection of the entire nidus is imperative as recurrences are likely when resection of the nidus is incomplete. Alternatively, an osteoid osteoma can be removed percutaneously by drilling the cortical bone involved under CT guidance. More recently, percutaneously performed radiofrequency ablation of osteoid osteomas has been employed. This was the treatment option utilized for this patient. The provided axial CT images demonstrate a radiofrequency probe extending completely through the lesion. After recovering from the conscious sedation utilized during the procedure, the patient reported complete resolution of his characteristic osteoid osteoma pain. The patient remains symptom-free at the time of this report, one month following the procedure.

Reference:
Resnick D, et al. Tumors and Tumor-Like Lesions of Bone: Imaging and
Pathology of Specific Lesions. In: Diagnosis of Bone and Joint Disorders, 3rd Edition.
Eds. Resnick D. W. B. Saunders, Co., Philadelphia; 1995(6):3633-3647.

Weissleder R, Rieumont MJ, Wittenberg J. Primer of Diagnostic Imaging, 2nd Edition.
Mosby, St. Louis; 1997:389.

Sartoris DJ. Musculoskeletal Imaging. The Requisites. Mosby, St. Louis;
1996:205-206.

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