Case Reports

Abdominal Wall Schwannoma

Fed Pract. 2019 March;36(3):129-133

References

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Pathology

On gross pathology examination, schwannomas have a well-circumscribed smooth external surface. On microscopy, schwannomas are truly encapsulated, uninodular, spindle-cell proliferations arranged in a streaming pattern within a background of thick, hyalinized blood vessels. Classic schwannomas typically exhibit a biphasic pattern of alternating areas of high and low cellularity and are named for Swedish neurologist Nils Antoni. The more cellular regions are referred to as Antoni A areas and consist of streaming fascicles of compact spindle cells that often palisade around acellular eosinophilic areas of fibrillary processes known as Verocay bodies.

In contrast, the lower cellularity regions (Antoni B areas) consist of multipolar, loosely textured cells with abundant cytoplasm, haphazardly arranged processes, and an overall myxoid appearance.¹¹ Schwannomas are known to have widely variable proportions of Antoni A and Antoni B areas; in this case, the excised specimen was noted to have predominately Antoni A areas without well-defined Verocay bodies and only scattered foci showing some suggestion of the hypocellular Antoni B architecture (Figure 2).^9,12

Immunohistochemical stains for S100 and SOX10 (used to identify cells derived from a neural crest lineage) were strongly positive, which is characteristic of schwannomas.¹³ Although there have only been rare reports of extracranial schwannomas undergoing malignant transformation, it is critical to rule out the possibility of a de novo malignant peripheral nerve sheath tumor (MPNST).¹³ In general, MPNSTs tend to be more cellular, have brisk mitotic activity, areas of necrosis, hyperchromatic nuclei, and conspicuous pleomorphism. Mitotic figures, which can be concerning for malignant potential if present in high number, were noted occasionally in our patient; however, occasional mitosis may be seen in classic schwannomas. Clinically, MPNSTs have a poor prognosis. Based on case reports, disease-specific survival at 10 years is 31.6% for localized disease and only 7.5% for metastatic disease.¹⁴ In this case, there was no evidence of any of the high-grade features of a malignant peripheral nerve sheath tumor, thus supporting the diagnosis of schwannoma (neurilemmoma).

Treatment

Schwannomas are exclusively treated by excision. Prognosis is good with low recurrence rates. It is unknown what the recurrence rates are for completely resected abdominal wall schwannomas since there are so few reports in the literature. For other well-known entities, such as vestibular schwannoma (acoustic neuromas), the recurrence rates are generally 2% to 3%.¹⁵Transformation of schwannomas into MPNSTs are so unusual that they are only described in single case reports.

Conclusion

Soft-tissue masses are a common complaint. Most are benign and do not require excision unless it interferes with the quality of life of the patient or if the diagnosis is uncertain. It is important to be aware of schwannomas in the differential diagnosis of soft-tissue masses. Diagnosis may be achieved through the combination of imaging and biopsy, but the definitive diagnosis is made on complete excision of the mass.

Acknowledgments
Contributors: Michael Lewis, MD, Department of Pathology, VA Greater Los Angeles Healthcare System. Written permission also was obtained from the patient.