Presentation
Seven year history of a palpable mass in the left calf. The appearance of an additional mass prompted the patient to seek care.
Patient Data
Ultrasound revealed two well circumscribed ovoid masses with a heterogeneously hypoechoic appearance located within the musculature. Hyperechoic rim is consistent with a capsule encasing the masses.
Doppler additionally shows increased vascularity within the masses.
Coronal images display soft tissue nodules in the lateral margin of the gastrocnemius muscle. The larger nodules more proximal in a location approximately 7.5cm below the fibular head measuring 1.6x1.0 cm in size. The second nodule is smaller and more distally in the left mid-leg approximately 13.5cm below the fibular head measuring 1.0 x 0.7cm in size. Similar appearing 7mm mass was found in the medial head of the gastrocnemius.
T1 coronals reveal a thin rim of hyperintensity at the proximal and distal poles of the masses representing a layer of fat.
Axial post gadolinium cross sections surrounding the largest mass have heterogeneous uptake with a slight predilection for the periphery.
Axial T2 fat sat imaging demonstrates small round hypointense regions in the center of the largest mass in groupings akin to fascicles of a nerve bundle.
Case 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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