A case of proximal-type epithelioid sarcoma arising in the base of skull

Abstract

Epithelioid sarcoma is a rare soft tissue tumour recently shown to display a characteristic loss of INI1 expression. It is subdivided into conventional (distal) and proximal forms, the latter showing large epithelioid carcinoma-like and/or rhabdoid cells. Epithelioid sarcoma rarely arises in the head and neck region, with only seven cases reported in an article by the French Sarcoma Group. In this report, we detail an instance where a proximal-type epithelioid sarcoma affected the skull base with involvement of the frontal lobe of the brain as well as extension into the nasal cavity of a 55-year-old Malay man. The excision specimen revealed a malignant epithelioid tumour forming sheets and lobules. Rhabdoid cells were prominent. The tumour cells showed widespread loss of nuclear expression of INI1, strong diffuse positivity for cytokeratin Cam 5.2 and epithelial membrane antigen, and focal positivity for synaptophysin, chromogranin and vimentin. To the best of our knowledge, this is the first case of a proximal-type epithelioid sarcoma arising from the base of skull and the second case arising from the bone. The histological differential diagnoses of skull base involvement by proximal type epithelioid sarcoma and its pathology are presented.