A Retrospective Analysis of High Resolution Ultrasound Evaluation of the "Split Fat Sign" in Peripheral Nerve Sheath Tumors

Peripheral nerve sheath tumors (PNST) comprise schwannomas and neurofibromas. The finding of increased adipose tissue around benign PNSTs has been described as the "split fat sign" on magnetic resonance imaging exams, which is suggestive of an intramuscular or intermuscular location of the tumor. However, few studies have described this sign as a salient ultrasound feature of PNSTs. The main purpose of this study was to retrospectively evaluate the presence of increased fatty tissue deposition around benign PNSTs diagnosed by high-resolution ultrasound. In addition, we aimed to corroborate the presence of vascularization around the affected area. A retrospective analysis of ten cases of PNSTs and two cases of post-traumatic neuromas diagnosed by high-resolution ultrasound was performed with a Logiq® P8 ultrasound with a 2-11 MHz multifrequency linear probe L3-12-D (central frequency: 10 MHz). Localized types of neurofibromas and schwannomas in any location were seen as predominantly hypoechoic tumors with an oval or fusiform shape. Exiting and entering nerves (tail sign) were observed in six cases, showing localized lesions both in intermuscular and subcutaneous locations. The presence of increased hyperechoic tissue (the split fat sign) was noted in cases of solitary intermuscular and intramuscular peripheral nerve sheath tumors, mainly the schwannomas. Though small tumors did not demonstrate the tail sign, the increase in adipose tissue and vascularity on US was well demonstrated. In conclusion, the nerve in continuity forms the basis of the ultrasonographic diagnosis of PNSTs. However, high-resolution US can convincingly demonstrate the increased presence of fat in the upper and lower poles as well as circumferentially in intermuscular or intramuscular benign PNSTs.