Objective: To analyse the MR imaging appearances of a large series of osteoid osteomas, to assess the ability of MR imaging to detect the tumour, and to identify potential reasons for misdiagnosis.
Design and patients: The MR imaging findings of 43 patients with osteoid osteoma were reviewed retrospectively and then compared with other imaging modalities to assess the accuracy of MR localisation and interpretation.
Results: The potential for a missed diagnosis was 35% based solely on the MR investigations. This included six tumours which were not seen and nine which were poorly visualised. The major determinants of the diagnostic accuracy of MR imaging were the MR technique, skeletal location, and preliminary radiographic appearances. There was a wide spectrum of MR signal appearances of the lesion. The tumour was identified in 65% of sequences performed in the axial plane. The nidus was present in only one slice of the optimal sequence in 27 patients. Reactive bone changes were present in 33 and soft tissue changes in 37 patients.
Conclusion: Reliance on MR imaging alone may lead to misdiagnosis. As the osteoid osteoma may be difficult to identify and the MR features easily misinterpreted, optimisation of MR technique is crucial in reducing the risk of missing the diagnosis. Unexplained areas of bone marrow oedema in particular require further imaging (scintigraphy and CT) to exclude an osteoid osteoma.