MRI in diagnosis of osteoid osteoma of the proximal femur: a potentially deviating aspect. Description of a clinical case.

Osteoid osteoma constitutes 10-12% of all benign neoplasms of the bone. The tumor more frequently involves the male sex (male to female ratio 2.1:1) and it may be observed in all age groups, with evident predilection for the second decade of life. All of the skeletal segments may be affected, but the most frequent site is the long bones, in the diaphyseal, metaphyseal and more rarely epiphyseal regions. The lesion is characterized by an osteolytic area, the nidus, which is at times partially calcified, surrounded by an osteosclerotic zone that is more or less accentuated. Clinical suspicion and traditional radiography are essential in diagnostic orientation; usually, further imaging methods are also recommended, such as bone scan with Tc99, CT scan and MRI. This last method allows for easy localization of the lesion, although with a sensitivity that is less than that of the CT scan. Nonetheless, the finding, if not supported by clinical suspicion, may be dangerously deviating and it may orient diagnosis towards a more aggressive disease.