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Case of the Month: July 2023

posted: July 28, 2023

19-years-old female with left leg pain for 3 months. Diagnostic imaging revealed a 2 cm tumor at the peroneal nerve. A resection of the mass was offered and tissue was sent to pathology for evaluation.

View Final Diagnosis
Final Diagnosis: Epithelioid Schwannoma
Epithelioid schwannoma (ES), a rare variant of nerve sheath tumor composed predominantly or exclusively of epithelial-appearing Schwann cells. They are largely restricted to superficial soft tissue with the most common location being the extremities. Histologically, they are composed of tight clusters of epithelioid Schwann cells. The tumor is usually nodular lesion with tan to white-yellow cut surface. EHs have predominantly or exclusively epithelioid appearance consisting of irregular groups, tight clusters, cords/trabeculae, or individual epithelioid Schwann cells embedded in a myxoid to hyaline matrix. On low-power, cases can be either hypercellular or hypocellular. Some cases can show focal spindling with cells displaying vaguely palisaded nuclei. Other features of classic schwannomas included prominently hyalinized and ectatic vessels and cystic change can be found as well. In about 10-20% of cases, focal atypical features represented by increased mitotic activity or nuclear atypia (but with no cell necrosis) can be seen (i.e., atypical variants). By immunohistochemistry the tumor cells are usually negative for cytokeratin (AE1/E3), however in rare cases a focal or weak staining can be seen. S100 is strongly positive so as SOX10. H3K27ME is intact. INI1/ SMARCB1 expression has been observed to be lost in subset of cases and its significance and prognosis is unknown but it’s important to know that it can be seen in sporadic cases and cases with germ line mutation of syndromic epithelioid schwannomas.