HSS Ultrasound of the Month Case #94

Clinical History A 67 year old male presents with progressively worsening right knee pain in one pinpoint location.

On exam, an exquisitely tender palpable focal point is appreciated along the anterolateral aspect of the right knee.

Clinical concern was for a ganglion cyst.

A right knee MRI was requested for further evaluation.

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A diagnostic ultrasound examination of the right knee was subsequently ordered to better delineate between a ganglion cyst and varicosity.

Additionally, an ultrasound-guided aspiration was requested if indicated.

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Diagnosis: Peripheral Nerve Sheath Tumor

Peripheral nerve sheath tumors (PNST) develop from nerve sheaths external to the central nervous system.

PNSTs are predominantly benign; however, large tumors and those associated with neurofibromatosis 1 can demonstrate malignant transformation.

Benign PNSTs include schwannoma, neurofibroma (localized, diffuse cutaneous and plexiform), perineuroma and hybrid nerve sheath tumors.

Patients typically present with a painful soft tissue mass and may demonstrate associated focal neurologic symptoms.

On examination, a firm, non-compressible, painful lesion is appreciated.

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Discussion Imaging Findings: MRI: • Fusiform lesion with tapered ends and evidence of a nerve entering into and exiting out from the mass

• Lesions typically demonstrate hypo/intermediate signal intensity on T1 weighted imaging and hyperintensity on T2 weighted imaging • Target sign = Central hypointense T2 signal that enhances with peripheral non-enhancing T2 hyperintense signal

• Fascicular sign = T2 hypointense round structures, which are presumed to be nerve fascicles, within a T2 hyperintense lesion • Muscle denervation in the distribution of the affected nerve Ultrasound: • Hypoechoic lesion with posterior acoustic enhancement and intrinsic blood flow, which appears continuous with an adjacent peripheral nerve

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