Prevalence and diagnostic significance of fluid-fluid levels in soft-tissue neoplasms

To report the prevalence of fluid-fluid levels (FFLs) in soft-tissue tumours as demonstrated by magnetic resonance imaging (MRI) and the potential diagnostic relevance of this finding.

A retrospective analysis was performed of 726 consecutive patients (361 women, 365 men, mean age 47.6 years+/-20.1 SD) presenting with a soft-tissue mass over a 7-year period. All subjects underwent MRI and final diagnosis was based on biopsy/surgical resection, or clinical follow-up and characteristic imaging findings. The patients were divided according to the presence or absence of FFLs on T2-weighted (T2W) axial MRI and histological diagnosis (non-neoplastic, neoplastic benign, neoplastic malignant). Cases with FFLs were sub-categorized depending upon the proportion of tumour containing FFLs: <1/3, 1/3-2/3 and >2/3, in order to determine whether the proportion of FFLs was useful for differential diagnosis.

Twenty-four of the 726 (3.3%: confidence interval 2.1-4.9%) soft-tissue masses contained FFLs. One of the 24 (4.1%) was non-neoplastic (one ganglion), 12 (50.0%) were benign neoplasms (nine haemangiomas, two schwannomas, one hamartoma) and 11 (45.9%) were malignant neoplasms (one leiomyosarcoma, one liposarcoma, one malignant fibrous histocytoma, one mxyofibrosarcoma, two primitive neuroectodermal tumours, two synovial sarcomas, one spindle cell sarcoma, and two sarcomas not otherwise specified). The presence of FFLs did not help to differentiate benign from malignant neoplasms. Of the 12 benign neoplasms, 66.7% contained over two-thirds FFLs, the majority of which were haemangiomas. Of the 10 malignant neoplasms, all contained less than two-thirds FFLs: 20% had less than one-third, 80% had one to two-thirds FFLs.

The prevalence of FFLs in soft-tissue tumours referred to a specialist orthopaedic oncology unit is 3.3%. However, the presence of FFLs does not reliably distinguish benign from malignant neoplasms, although all lesions with more than two-thirds FFLs were benign.