The prevalence and diagnostic significance of fluid-fluid levels in focal lesions of bone

Objective. To determine the prevalence and diagnostic significance of fluid-fluid levels (FFLs) in focal bone lesions. Design and patients. Clinical and radiological details of 738 consecutive patients referred with focal lesions of bone and who had undergone MRI were reviewed. FFLs were identified in 83 (11.2%). The proportion of the lesion occupied by FFLs was estimated, based on imaging in all available planes, as <1/3, 1/3-2/3, >2/3 but not the entire lesion, and complete. The degree of FFL change in each lesion was correlated with the final diagnosis, which was either histological (n=80) or clinicoradiological (n=3). There were 31 female and 52 male patients, mean age 25.5 years (range 5-83 years). Results. Histology revealed 46 benign, 32 malignant and 2 non-neoplastic lesions. A clinicoradiological diagnosis was made in the 3 lesions without histology: 2 were benign (simple bone cyst and intraosseous lipoma) and 1 malignant (a metastasis). Malignant neoplasms commonly showed FFLs which occupied <1/3 of the entire lesion (n=22/32, 68.8%), and 50% of all the lesions in this group were conventional intramedullary osteosarcomas (n=16). With increasing FFL change, malignancy became less frequent: with >2/3 (but incomplete) FFL change, 81% (n=13/16) of tumours were benign. If the entire tumour showed FFL change, the histology was benign in 100% (n=11). Conclusions. The extent of FFLs within a focal bone lesion appears to be inversely related to the degree of malignancy. If at least 2/3 of the lesion shows FFL change, 89% of diagnoses are benign.