The 'split fat sign' revisited

This review article revisits the 'split fat sign', reviewing its aetiology and prevalence in benign and malignant soft tissue tumours in both the intermuscular and intramuscular locations. The term 'split fat sign' was first used in 1999, referring to the presence of a rim of fat surrounding neurogenic neoplasms in the intermuscular space on magnetic resonance imaging (MRI). The presence of fat in the intermuscular plane is a normal finding and therefore the 'split fat sign' can be seen in both benign and malignant intermuscular lesions. The term 'split fat sign' has also been used to refer to the presence of perilesional fat on MRI surrounding intramuscular tumours. Other terms have also been used to describe the finding in association with intramuscular tumours, including 'fatty rind' and 'peritumoral fat'. Although typically associated with benign tumours, the presence of perilesional fat has also been described in multiple cases of malignant intramuscular lesions. The radiologist must be aware that the presence of perilesional fat surrounding a deep-seated soft tissue tumour is not diagnostic of a benign lesion. Whenever there is diagnostic uncertainty, cases should be reviewed at a soft tissue sarcoma centre to ensure appropriate ongoing management.