The incidence and relevance of non-fatty components in trunk and extremity lipomatous soft tissue masses.

To determine the incidence and diagnostic relevance of non-fatty 'solid appearing' components within lipomatous tumours of the trunk and extremity. Retrospective review of patients referred to a specialist musculoskeletal oncology service over a 12-month period with a lipomatous trunk or extremity soft tissue tumour. The presence and morphology (solitary/multifocal; homogeneous/heterogeneous; well-defined/poorly defined) of non-fatty components was recorded based on MRI and compared with the final histological diagnosis. 213 patients with 217 lipomatous tumours were included, 119 (55.9%) males and 94 (44.1%) females with mean age of 54.6 years (range 7-93 years). Seventy-seven (35.5%) lesions arose superficial to the fascia and 139 (64.1%) deep, while a single case involved both compartments. Mean maximal tumour dimension was 94.9 mm (range 12-288 mm). Non-fatty 'solid appearing' components were identified in 28 (12.9%) cases, of which eight were solitary and 20 were multifocal, six had homogeneous SI and 22 had heterogeneous SI, and eight had well-defined margins, while 20 had poorly defined margins. Histological diagnosis was available in 20 of the tumours containing non-fatty components, 16 of which were benign, two intermediate grade and two malignant (a dedifferentiated liposarcoma and a myxoid liposarcoma). The commonest diagnosis was spindle cell lipoma, which accounted for 10 of 20 (50%) cases with confirmed histology. Non-fatty components are identified in ~13% of trunk and extremity lipomatous tumours. The majority of such lesions are benign lipoma variants, most commonly spindle cell lipoma. Solid non-fatty components are identified in approximately 13% of lipomatous tumours referred to a specialist sarcoma service. Despite the concern that these may represent dedifferentiated liposarcomas, high-grade tumours were seen in only two cases, the commonest diagnosis being a spindle cell lipoma.