Peripheral Nerve Sheath Tumor Accession # 108167

1. James Montgomery, DVM December 29, 2008 Peripheral Nerve Sheath Tumor Accession # 108167

2. Accession # 108167 • Boomer • 9 year old, MC, Portuguese water dog • 11 month history of left front lameness • Intermittent non-weight bearing • Non-responsive to NSAIDs or acupuncture • Survey radiographs – WNL • Left shoulder arthrogram (Acc #107751) - WNL

3. Biceps brachii tendon is smooth and regularly marginated.  No filling defects are identified within the joint space. The infraspinatus tendon is also outlined by contrast medium; it appears smoothly marginated.  Left shoulder arthrogramAccession # 107751

4. MRIAccession # 108167 • 1 cm tubular mass caudomedial to the left shoulder joint • Extends dorsomedially and appears contiguous with thickened 7th and 8th cervical spinal nerves • Apparent extension into the vertebral canal on the left at C7 – T1 • Findings consistent with malignant peripheral nerve sheath tumor

5. Malignant Peripheral Nerve Sheath Tumor • Have been referred to as schwannomas, neurinomas, neurilemmomas, neurofibromas, and neurofibromas • Most canine PNST are poorly differentiated • Cell origin is difficult to identify • May arise from any peripheral nerve • 80% - brachial plexus or its nerve roots • 20% - pelvic plexus, thoracolumbar nerve roots, and cranial nerves (trigeminal most commonly affected CN)

6. Malignant Peripheral Nerve Sheath Tumor • Typically slow growing • Extend by local invasion along peripheral nerve roots and its branches • Rarely invade surrounding tissues • Metastasis rare • Neurologic signs due to compression of peripheral axons, spinal cord, or brain stem

7. Malignant Peripheral Nerve Sheath Tumor • Signalment: • Mature dogs, rarely seen in cats • No breed or sex predilection • Clinical signs: • Chronic, progressive forelimb lameness • Muscle atrophy • Pain in the axillary area • Palpable mass • Horner’s syndrome, with loss of ipsilateral panniculus – loss of the first 2 thoracic nerve roots (contain preganglionic sympathetic fibers supplying the face, and give rise to the lateral thoracic nerve)

8. Malignant Peripheral Nerve Sheath Tumor • Clinical signs continued: • LMN signs to affected limb • May be hemiparetic • Early signs very much like orthopedic disease • Differentiating between orthopedic and neurologic disease can be difficult • May result in a long period before a diagnosis is made • Between 4.8 and 6.1 months in one report

9. Malignant Peripheral Nerve Sheath Tumor • Imaging • Survey radiographs – usually normal • Rule out orthopedic disease • May show enlarged intervertebral foramen or occasionally vertebral body lysis • Myelography • May see intradural extramedullary mass if nerve roots have been invaded • Ultrasound • Good screening tool but lack of tumor identification doesn’t rule out presence of a tumor • With severe muscle atrophy and no orthopedic disease, CT or MRI should be performed

10. Ultrasound • Hypoechoic tubular axillary mass with no blood flow • Limitations • Difficulty in detecting extent of tumor • Measurements do not correlate with size of tumor • Axillary lymph nodes – false positive diagnosis

11. CT • Rim enhancement noted frequently, often with central hypoattenuating areas • Masses as small as 1.0 cm can be identified on contrast enhanced scans • Smaller mass-like lesions may not be specifically associated with neuronal structures

12. CT • Relies on noticeable soft tissue asymmetry or contrast enhancement • Can be difficult to assess nerve structures oriented obliquely to the transverse plane • MRI superior to CT for detecting brachial plexus tumors • Excellent contrast resolution • Able to distinguish nerve bundles from vessels

13. MRI – Kraft, et al. 2007 • Majority – hyperintense to muscle on T2W images and isointense on T1W images • Most only minimally or heterogeneously contrast enhancing • Contrast enhancement critical to detecting subtle diffuse nerve sheath involvement or small isointense nodules • Transverse plane images including both axillae and the vertebral canal allow in-slice comparison to detect lesions by asymmetry

14. MRI – Kraft, et al. 2007 • Higher resolution, smaller field of view, multiplaner examination of the cervicothoracic spine important for appreciating nerve root and foraminal involvement • Findings: • ~1/2 had diffuse thickening of brachial plexus nerves (smooth or nodular) • ~3/4 with diffuse thickening had extension into the vertebral canal

15. MRI – Kraft, et al. 2007 • Findings (continued): • Nerve root thickening associated with widening of the intervertebral foramen • Contrast enhancement was only mild to moderate, and usually non-uniform • Ipsilateral muscle atrophy with hyperintense muscle bundles – T2, STIR, and pre-contrast T1 • Neurogenic atrophy, edema, fatty infiltrate, and fibrosis

16. MRI – Kraft, et al. 2007 • Transverse precontrast (A) and postcontrast (B) T1-weighted images • Mass is isointense to muscle, and is identifiable by absence of a similar structure in slice images of the contralateralaxilla. • Postcontrast -minimal heterogeneous contrast enhancement

17. MRI – Kraft, et al. 2007 • PD sequences did not provide unique info • T2W – lesions hyperintense to muscle but less intense than axillary fat • T2W – STIR – suppresses fat making hyperintense neoplastic lesions more noticeable • Small or diffuse lesions particularly difficult to see due to similar intensity to fat on T2W images, and to muscle on T1W images (pre-contrast) • As with people, visible contrast enhancement on T1W images is critical to detecting canine peripheral nerve sheath tumors

18. Treatment and outcome • Early diagnosis and surgical removal key to good outcome • Canine PNST have a more infiltrative and malignant behavior than the human counterpart • Tumor margins difficult to identify grossly • Aggressive surgery recommended • Tumor recurrence common • Median survival • Brachial plexus tumor – 12 months • Nerve roots – 5 months • If nerve root and vertebral canal involvement, poor outcome

19. References • Inzana KD. Peripheral Nerve Disorders. In Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine, 6th ed (St. Louis, MO: Elsevier Saunders, 2005) pp. 896-7. • Kraft S, et al. MRI characteristics of peripheral nerve sheath tumors of the canine brachial plexus in 18 dogs. Vet Radiol Ultrasound 48(1):1-7;2007. • Platt SR, et al. MRI and ultrasonography in the diagnosis of a malignant peripheral nerve sheath tumor in a dog. Vet Radiol Ultrasound 40(4):367-71;1999. • Rose S, et al. Ultrasonographic evaluation of brachial plexus tumors in 5 dogs. Vet Radiol Ultrasound 46(6):514-7;2005 • Rudich SR, et al. CT of masses of the brachial plexus and contributing nerve roots in dogs. Vet Radiol Ultrasound 45(1):46-50;2004.