Synchronous soft-tissue tumors with the split fat sign on MRI: a case report of ancient schwannomas

Abstract Background The “split fat sign” is a magnetic resonance imaging (MRI) feature characterized by a thin rim of fat surrounding a soft-tissue mass, best appreciated on T1-weighted images. It is typically associated with well-circumscribed, slow-growing lesions and is most commonly seen in benign peripheral nerve sheath tumors. The aim is to describe the magnetic resonance imaging findings of benign neurogenic tumors and discuss the differential diagnosis with other soft-tissue tumors that may also show the split fat sign. Case presentation We presented a 48-year-old male who was referred to our department to evaluate two deep painless soft-tissue masses in the right calf and foot. In MRI, both lesions showed well-defined encapsulated margins and the “split fat sign”. The calf mass showed hyperintense signal like cystic mass, and the medial plantar mass showed a fascicular sign on T2-weighted images. The biopsy of two lesions with S100 antigen positivity confirmed ancient schwannomas. Conclusion The relation of the “split fat sign” with benign peripheral nerve sheath tumors was confirmed, as well as the cystic appearance on T2 in a variant of schwannoma known as ancient schwannoma. Despite the advantages of MRI, the biopsy is essential for a definitive diagnosis.