CASE 116

By: Dr. Nizar Al-Nakshabandi MD, FRCPC

HISTORY: 39-year-old male with a lump posterior to his knee.

FINDINGS:

Sagittal gradient echo T2* of the right knee and distal femur demonstrate a soft tissue mass posterior to the distal femur measuring 4 x 3 x 2 cm of high signal intensity and heterogeneous with areas of low signal intensity centrally representing hemosiderin and giving the salt and pepper sign. Note that the lesion is attached to a cordlike structure inferiorly and superiorly likely to be present a peripheral nerve. Also note that the lesion splits the fat superiorly consistent with the split fat sign.

Axial T1-weighted images and T2-weighted images [B and C respectively] demonstrate that the lesion is homogeneously low signal intensity on T1-weighted image [B] and heterogeneously high signal intensity on T2-weighted images with once again what appears to be the Salt and pepper sign.

Coronal Postcontrast T1-weighted images with fat saturation [D] demonstrate avid enhancement of the lesion with central areas of low signal intensity that do not enhance. Again, note is made of the split fat sign superiorly.

DIFFERENTIAL DIAGNOSIS:

·        Schwannoma of the peripheral nerve.

·        Neurofibroma.

·        Sarcomas from mesenchymal region could be considered but appears to be less likely and do not have the same signal characteristics and signs.

DIAGNOSIS:

Schwannoma of the peripheral nerve.

PEARLS AND DISCUSSION:

Schwannomas are benign tumors of peripheral nerves and can be seen anywhere. Intracranial schwannomas can involve any cranial nerve except the olfactory and optic nerve. Spinal schwannomas involve spinal nerves. In the trunk, they have affinity to involve the intercostal nerves but can be seen in the posterior mediastinum and gastrointestinal tract. Furthermore, in the limbs they favor the flexor surfaces of the limbs.

Most schwannomas are solitary [90%]. However, there is an association with neurofibromatosis type II. Multiple schwannomas occur in neurofibromatosis-2.

They are well-circumscribed bright lesions on T2-weighted images due to the cystic and fatty degeneration. They show heterogeneity because of the cystic degeneration or hemorrhage. When they hemorrhage you can see the hemosiderin within them. The split fat sign can be seen on MRI images.

The salt and pepper appearance of multiple ring-like structures with peripheral hyperintensity on T2, although this sign can be present in any neurogenic tumour. However, in 15% of schwannomas, may instead have a target sign i.e. hyperintense rim of myxoid tissue with central hypointense area due to more fibrocollagenous tissue on T2-weighted imaging. Finally, when contrast is injected, schwannomas usually display inhomogenous and diffuse enhancement.

These lesions are quite painful on percutaneous biopsy and should be approached with caution when asked to be biopsied under CT or ultrasound guidance. Good infiltration of the skin and subcutaneous tissue with Xylocaine is necessary. In addition, since there is neurological origin conscious, sedation may be needed at the time of biopsy.

Surgical excision is the treatment of choice. Malignant transformation almost never happens.

FURTHER READING:

1.      Beaman FD, Kransdorf MJ, Menke DM. Schwannoma: radiologic-pathologic correlation. Radiographics. 24 (5): 1477-81.

2.      Weissleder R, Wittenberg J, Harisinghani MG. Primer of diagnostic imaging. Mosby Inc. (2003) ISBN:0323023282.

3.      Murphey MD, Smith WS, Smith SE et-al. From the archives of the AFIP. Imaging of musculoskeletal neurogenic tumors: radiologic-pathologic correlation. Radiographics. 19 (5): 1253-80