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Neuroradiology Case of the Week

Case 326

Salman Mirza, DO, Balasubramanya Kolar, MD
and P-L Westesson, MD, PhD, DDS

Clinical Presentation: Patient is a 25-year-old female who presented with right leg pain in S1-S2 nerve root distribution.

Imaging Findings: A well-defined T1 isointense,T2 hyperintense, diffusely enhancing dumbbell-shaped mass lesion arising from right sacral (S2) nerve root was noted, expanding the neural foramen on right at S2-S3 level. Bony remodeling was noted without any destruction.

Diagnosis: Sacral neurofibroma

Discussion: Neurofibromatosis type 1 (NF1) is the most common of the phakomatoses and has a variety of localized or, more frequently, systemic manifestations throughout the thorax, abdomen, pelvis, and extremities [1]. Sacral lesions commonly occur in NF1 due either to direct involvement by neurofibromas or to remodeling caused by dural ectasia [2]. Simple enlargement of neural foramina is rather characteristic for NF1 but may also be observed in conditions such as Marfan syndrome. When bone destruction occurs, lesions often appear more aggressive and can simulate other sacral lesions such as chordoma, lymphoma, giant cell tumor, and teratoma.
     These are benign fibroblastic neoplasms of peripheral nerves whose consistency and histologic appearance vary from myxoid to fibrous according to the differentiation of
the neoplastic elements [3].
     These tumors arise from the nerve sheath and therefore have a fusiform shape in contrast to schwannomas which are eccentric. Since they are benign, slow growing lesions, they do not cause bony destruction. Usually, bony remodeling occurs with scalloping or enlargement of the foramina [4].
     Surgical resection and debulking of the tumor is the primary treatment and sparing the adjacent nerve root is essential for a good prognosis [5]. Nerve sheath tumors make up only a small percentage of the wide variety of lesions that occur in the sacral region. These tumors can reach extremely large size prior to producing any significant neurological symptoms. Most nerve sheath tumors in the sacral region are schwannomas.
     Pain is the most common presenting symptom which includes low back or radicular pain which extends in the distribution of one or more lumbar/sacral roots. Other presenting symptoms include urinary retention, cystitis, and sensory paresthesias.
     The majority of sacral nerve sheath tumors are schwannomas. Schwannomas are benign tumors composed entirely of Schwann cells. The classic histological appearance are spindle shaped cells with eosinophilic cytoplasm either densely packed (Antoni A) or less cellular and loosely textured (Antoni B). Schwannomas can demonstrate fatty degeneration and cystic change. Hemorrhage is not uncommon either. Schwannomas typically present in the 3rd through 6th decade of life. There is an equal male to female distribution.
     Sacral nerve sheath tumors can also be a neurofibroma. These benign tumors have a polymorphic cellular makeup. Neurofibromas include Schwann cells, perineural cells, and fibroblasts. The cells are widely spaced on histology with thin, elongated nuclei and minimal cytoplasm.
     Malignant nerve sheath tumors are very rare. The main features of these tumors include intrafascicular spread and invasion of surrounding soft tissue. Histology demonstrates high cellularity with multiple mitoses and areas of necrosis. Prior radiation therapy has been shown to contribute to the formation of malignant nerve sheath tumors.
     Surgery is the treatment of choice for sacral schwannomas. However, complete resection is often difficult due large size, abundant vascular size, and proximity to neurological structures. Radiotherapy is usually avoided in treatment of these benign tumors given the risk of developing secondary neoplasms. However, recently stereotactic radiosurgery has become an alternative, primarily in tumors less than 3 cm in size.

References:

1. Klimo P Jr, Rao G, Schmidt RH, Schmidt MH. Nerve sheath tumors involving the sacrum. Case report and classification scheme. Neurosurg Focus. 2003 Aug 15;15(2):E12. [Medline]
2. Fortman BJ, Kuszyk BS, Urban BA, Fishman EK. Neurofibromatosis type 1: a diagnostic mimicker at CT. Radiographics. 2001 May-Jun;21(3):601-12. [Medline]
3. Ito Y, Fukumura A, Urasaki E, Ushio Y. A case of neurofibromatosis with spinal dural ectasia and vertebral body scalloping: a case report. No Shinkei Geka. 1988 Dec;16(13):1495-9. [Medline]
4. Harkin JC, Reed RJ. Tumors of the peripheral nervous system. Washington, DC: Armed Forces Institute of Pathology, 1969:51-97.
5. Barboriak DP, Rivitz SM, Chew FS. Sacral neurofibroma. AJR Am J Roentgenol. 1992 Sep;159(3):600. [Medline]
6. Feldenzer JA, McGauley JL, McGillicuddy JE. Sacral and presacral tumors: problems in diagnosis and management. Neurosurgery. 1989 Dec;25(6):884-91. [Medline]