Peripheral nerve sheath tumor

Discussion:

This case demonstrates classic findings of peripheral nerve sheath tumors on ultrasound as well as some characteristic signs on MRI. 

On the initial ultrasound evaluation, these masses embedded in the musculature of the calf were well circumscribed, heterogeneously hypoechoic, and displayed increased vascularity. All of these findings are well correlated with a PNST 1 but are not entirely specific so additional characterization with MRI was performed. 

Most notably, the MRI exhibited a "split fat" and "fascicular" sign consistent with PNST. The split fat sign is most notable on the T1 coronals and is characterized by the thin rim of hyperintensity around the masses that tapers at the proximal and distal ends 2. The fascicular sign is best appreciated on the axial T2 fat sat images in which there are small, stacked, ringlike hypointensities in the center of the mass 2.

The combination of the split fat and fascicular signs represents a neurogenic origin of the tumors and the lack of multiple malignant traits (diameter of the mass greater than 5 cm, peripheral enhancement on T1+C, perilesional edema on T2, and intramural cystic lesions) makes benign peripheral nerve sheath tumors the most likely diagnosis 3. Schwannomas and neurofibromas are two of the most common types; however, imaging is not a reliable means of distinguishing between the two and histological evaluation is needed for confirmation. A biopsy was not performed in this case due to the patient's preference and conservative management with watchful waiting was initiated. Biopsy and surgical evaluation would be considered should the masses rapidly increase in size or if neurologic symptoms developed. 

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