October 2013 Case 1

A 28 year old female with back pain and lower extremity weakness.

Yara Daous, M.D., Tiffany Harper, M.D.

Overview

A 28 year old female presented with progressively worsening back pain and lower extremity weakness and numbness associated with loss of sensation and difficulty voiding over a period of two weeks.

Gross Description

A lumbar spine computed tomography scan showed a soft tissue mass in the left L4-L5 neural foramen with erosion of the posterior aspect of the L4 vertebral body.

A magnetic resonance imaging study of the lumbosacral spine showed a dumbbell configuration tumor at the L4-L5 level with adjacent gross destructive change and abnormal enhancement in the L4 vertebral body.

An L4 vertebral bone biopsy showed:

Image #1, bone biopsy 4X.
Image #2, bone biopsy 20X.
Image #3, HMB-45 4X.
Image #4, S-100 4X.
Image #5, type IV collagen 40X.

A cerebrospinal fluid cytology was negative for malignant cells and showed mixed inflammatory cells and red blood cells.

The patient then underwent a L3-5 laminectomy for tumor. This showed:

Image #6, Lumbar vertebral body with tumor, resection.
Image #7, L3-L4 nerve roots with tumor, resection.
Image #8, lumbar spine with tumor 2X.
Image #9, tumor 40X.
Image #10, tumor to L3-L4 nerve roots, 2X.

A PET CT scan showed no evidence of metastasis.

A thorough skin examination showed no atypical lesions.

Diagnosis

What is the most likely diagnosis?

Answer

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Discussion

Melanotic schwannoma is a rare pigmented variant of schwannoma. It occurs at an earlier age than conventional schwannomas. It appears grossly circumscrimbed but is not encapsulated. The tumor is composed of melanin-producing Schwann cells giving it a black or brown pigmentation. Two different forms exist: nonpsammomatous and psammomatous.

Nonpsammomatous lesions more commonly involve the spinal nerve roots or nerve ganglia, but can also affect cranial nerves. They can also be intraparenchymal. Lesions affecting nerve roots commonly have a dumbbell configuration. The reason for acquiring such configuration is that as these tumors become larger, they tend to protrude out of the corresponding neural foramen, resulting in a dumbbell shaped mass. Microscopically, nonpsammomatous melanotic schwannomas are hypercellular and are composed of spindle and epithelioid cells arranged in fascicles, whorls, or lobules. Cells have a round nucleus and a prominent nucleolus. They can be multinucleated, may have nuclear-cytoplasmic pseudoinclusions, and have variable melanin pigmentation.

Psammomatous lesions often involve the autonomic nerves of the viscera, specially the gastrointestinal tract and the heart. These tumors may show gross calcification and bone metaplasia. Microscopically, psammomatous melanotic schwannomas have abundant psammoma bodies, and cells tend to have large cytoplasmic vacuoles resembling adipocytes. Approximately 50% of psammomatous melanotic schwannomas occur in individuals with Carney's complex, which is an autosomal-dominant disorder. It is characterized by lentiginous pigmentation predominantly involving the face and external genitalia, heart myxomas, blue nevi, and endocrinopathies like precocious puberty and acromegaly. Multifocality occurs in 20% of the psammomatous lesions. Tumors arising in patients with Carney's complex have been noted to have allelic loss of PRKAR1A region on 17q.

Most melanotic schwannomas are benign but approximately 10-15% are malignant. These malignant tumors tend to invade through soft tissue and destroy surrounding bone. They have severe cytologic atypia, increased mitotic activity including atypical forms, and geographic areas of necrosis.

Due to the presence of melanosomes, melanotic schwannomas are immunoreactive with melanoma markers such as S-100 protein and HMB-45, making the destinction sometimes difficult. Positive membrane staining for collagen IV and laminin is seen in all schwannomas. In melanotic schwannomas, collagen IV staining tends to surround nests of cells rather than the usual pericellular pattern.

Ultrastructurally, these tumors consist of irregular spindle cells containing melanosomes, thin interdigitating cytoplasmic processes, and have less uniform lining by basal lamina than conventional schwannomas.

References

  1. Font RL, Truong LD (1984). Melanotic schwannoma of soft tissues. Electron-microscopic observation and review of literature. Am J Surg Pathol 8: 129-138.
  2. Edward A, Bermudez C, Piwonka G, Berr ML, Zamorano J, Larrain E, Frank R, Gonzalez M, Alvarez E, Maiers E (2002). Carney's syndrome: complex myxomas. Report of four cases and review of the literature. Cardiovasc Surg 10: 264-275.
  3. Carney JA (1990). Psammomatous melanotic schwannoma. A distinctive, heritable tumor with special associations, including cardiac myxoma and Cushing syndrome. Am J Surg Pathol 14: 206-222.
  4. Scheithauer BW, Woodruff JM, Erlandson RA (1999). Tumors of the Peripheral Nervous System. Armed Forces Institute of Pathology: Washington, D.C.