Characterisation of intra-articular soft tissue tumours and tumour-like lesions.

The aim of this study was to describe a new magnetic resonance imaging (MRI) classification system for intra-articular soft tissue tumours based on the morphology of the lesion, with the aim to aid the differential diagnosis. We performed a retrospective review of 52 consecutive patients presenting to a specialist musculoskeletal oncology unit with a suspected intra-articular tumour. Lesions were categorised into one of four groups according to a simple classification system based on their morphological features on MRI. Distinct groupings of pathologies emerged corresponding to each of the morphological categories. Particularly when combined with radiographic features of calcification and bone erosion, certain patterns were found to be characteristic of specific diagnoses. For example multifocal, calcified lesions were found exclusively in synovial osteochondromatosis and diffuse synovitis with hypointense T2-weighted signal intensity was typical of pigmented villonodular synovitis. Certain combinations of imaging features such as diffuse solid lesions and focal lesions with bone erosion were commonly associated with malignant lesions. We suggest that by classifying intra-articular masses according to their morphological features on MRI, particularly when combined with simple radiographic features, an additional parameter may be generated to aid the radiologist in making a diagnosis. In addition, particular combinations of features provide 'red flags' to increase the index of suspicion for malignancy.