Presentation
Small palpable mass in the medial aspect of the left distal thigh with numbness and tingling.
Patient Data
Well-defined ovoid heterogeneously hypoechoic intramuscular mass within the vastus medialis muscle with linear hypoechoic and hyperechoic structures representing most likely the fascicular bundles within the nerve. Internal vascularity is noted within the mass. A thin hyperechoic rim around the mass representing a layer of fat.
On the MRI, there is a well-circumscribed mass of intramuscular location within the vastus medialis, isointense to muscle on T1 with a peripheral hyperintense rim "split fat sign" and hypointense centrally with surrounding hyperintense rim on T2 "target sign". Moderate enhancement is noted on postcontrast sequences.
Case Discussion
The clinical presentation and the ultrasound and MRI features are most consistent with a peripheral nerve sheath tumor.
Peripheral nerve sheath tumors (PNSTs) are a group of primary neurogenic tumors that arise from nerve sheaths outside of the CNS. The vast majority are benign, however, malignant transformation is seen particularly in large tumors and those associated with neurofibromatosis type 1 (NF1).
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