LAUSR

Background: Epithelioid sarcoma (ES) is a rare malignant mesenchymal tumor that only accounts for 0.6% to 1.0% of all cases of sarcomas. ES with a relative quiescent state of more than 10 years is extremely rare.Here, we present a rare case of ES in the forearm of a 17-year-old girl. The patient had a congenital mass in her forearm that measured approximately 1cm; it grew rapidly starting 5 years ago. The mass was not treated until last year when she underwent the first surgery. The mass was located in the middle and lower part of the left forearm and involved the dorsal muscle group, intermuscular space, and subcutaneous tissues without clear boundaries.The patient underwent surgery, and the tumor recurred twice within 1 year postoperatively. Methods: The tumor samples were examined via hematoxylin-eosin (HE) and immunohistochemistry staining. Results: Histopathologically, the tumor comprised large polygonal epithelioid cells with abundant eosinophilic cytoplasm arranged in cell nests. Central necrosis and focal myxoid change could be seen in the tumor tissues. Immunostaining showed that the tumor cells were positive for CD34, CK, EMA, and vimentin but negative for CD31, S-100, and INI-1. Conclusion: Based on these findings, the tumor was diagnosed as ES of distal form. Distal-type ES could have a long period of relative quiescence, after which it could grow rapidly and relapse multiple times over a short duration.