Benign epithelioid peripheral nerve sheath tumour, benign epithelioid Schwannoma

Epidemiology

Although an epithelioid variant of malignant Schwannoma is recognised, the existence of a benign counterpart has been questioned.  In 1998, a report of 5 cases was published2. Benign epithelioid neurofibroma is even rarer. A relatively large study found difficulty classifying these tumours as either Schwannoma or neurofibroma, preferring the term "benign epithelioid peripheral nerve sheath tumour"3.

Clinical features

This is a tumour found across a large age range and a large range of anatomical sites. Rare cases are associated with neurofibromatosis type I3.

Macroscopic appearances

The lesions usually occur in the dermis or subcutis and are small, superficial and well circumscribed.

Histopathology

Cellularity is moderate to high, with the tumour mainly consisting of epithelioid cells in loosely aggregated clusters. The cells vary from nevus like to large with eosinophilic cytoplasm. Tight syncytial aggregates mimic multinucleation. There is a stroma which variously may be collagenous, myxohyaline or pure myxoid. The cells may form rosettes around nodules of collagen. About half the cases have a spindle cell component. The mitotic rate is low, with insignificant cytological atypia and a lack of necrosis in most cases: a minority show enlarged hyperchromatic nuclei, prominent nucleoli and up to 6 mitoses /10 HPF3.

Epithelioid neurofibroma shows no capsule and a myxoid stroma: there may be permeating intra-neuronal growth

Epithelioid Schwannoma is encapsulated and shows thick-walled vessels, rosette like structures and strong positivity for S-100 but lacks clear Antoni A and B areas.

Immunohistochemistry

Differential diagnosis

• Epithelioid malignant PNST/Schwannomas show frank malignancy with brisk mitotic activity, frequent necrosis and marked cellular atypia.  They are usually deeper and larger. The superficial variant shows encapsulation and is composed of smaller cells, but also shows frankly malignant cytology. The small and superficial malignant lesions usually do not recur after excision.

• Glandular malignant Schwannoma contains true epithelial cells forming glandular structures.

• Benign superficial Schwannoma with entrapped adnexal sweat glands

• Other benign and malignant peripheral nerve sheath tumours.

• True epithelial neoplasms

• Myoepithelioma of soft tissue: positive for S-100 and GFAP but also for cytokeratins, p63, SMA and calponin.

• Epithelioid mesenchymal lesions

• Melanocytic lesions: positive for melanocytic markers

• Neurocristic hamartoma

• Cellular neurothekeomas: usually negative for S-100, NGFR and GFAP but may express MITF

• Ossifying fibromyxoid tumour of soft parts: the capsule and fibrous septa contain bone. The cells are more loosely arranged. Positive for S-100 and GFAP, but also often focally for desmin.

• Glomus tumour: negative for S-100 but positive for SMA, calponin and caldesmon.

Prognosis

Behaviour is benign, even in tumours with atypical features3.

References

1Kindblom L-G, Meiss-Kindblom J, Havel G, Busch C.   Benign epithelioid schwannoma, Am J Surg Pathol 1998;22:762-70.

2Advances in Anat Pathol 1999;6:59.

3Laskin, W. B., J. F. Fetsch, et al. (2005). "Benign Epithelioid Peripheral Nerve Sheath Tumors of the Soft Tissues: Clinicopathologic Spectrum of 33 Cases." Am J Surg Pathol 29(1): 39-51.

This page last revised 13.1.2005.

©SMUHT/PW Bishop