Metastatic proximal epithelioid sarcoma in pleural effusion: cytopathologic findings and differential diagnosis.

We present a rare occurrence of metastatic proximal epithelioid sarcoma (PES) in the pleural effusion of a 23-year-old man, developed within one year of diagnosis in his gluteal soft tissue. The cytologic and immunoperoxidase findings are described. PES, due to its epithelioid morphology, can be confused with more common cancers in effusions such as adenocarcinoma and mesothelioma. PES is an aggressive neoplasm that differs clinically and pathologically from conventional epithelioid sarcoma. Knowledge of its cytomorphology in serous cavity effusions, a patient's clinical history and ancillary studies may lead to an accurate diagnosis.