The incidence and significance of fluid-fluid levels on computed tomography of osseous lesions.

The demonstration of a fluid-fluid level (FFL) within an osseous lesion on computed tomography (CT) has been reported as suggestive of an aneurysmal bone cyst (ABC) although FFLS have also been rarely found in association with other lesions. This study was conducted to determine the frequency of FFLS on CT in a group of ABCs and a series of patients presenting to a major tertiary referral centre for the treatment of bone tumours. An FFL was present on CT in 21 (84%) of the 25 ABCs and in 17 was multiple. FFLs are typical of the mid ("blow-out") or late phase of development of an ABC and not the incipient ("permeative") stage or where the internal architecture of the tumour has been disrupted by biopsy or previous surgery. In a 3-year period, 16 ABCs were found in 491 bone lesions referred to a bone tumour treatment centre. CT of the ABCs revealed FFLs in 14 (87.5%) cases. Within the same period, 728 CTs of these and other bone lesions were performed and FFLs were identified in two further cases: a massive telangiectatic osteosarcoma and a conventional osteosarcoma following chemotherapy. The diagnostic significance of an FFL on CT for ABC is: sensitivity = 87.5%, specificity = 99.7%, positive predictive value = 87.5%, negative predictive value = 99.7%, accuracy = 99.4%. An FFL within a bone lesion on CT remains strongly suggestive of an ABC although the radiologist should be wary of a rare telangiectatic osteosarcoma.