Cytologic features of epithelioid gastrointestinal stromal tumor in a pleural effusion. A diagnostic challenge

Gastrointestinal stromal tumor (GIST) is the most common mesenchymal neoplasm of the gastrointestinal tract. Despite the possibility of peritoneal spread, few cytologic reports of GIST in effusions are available. In this report, we describe cytologic features of a case of abdominal epithelioid GIST metastatic to the pleura. To our knowledge, it is the first cytologic description of this tumor in such a location. The rarity of pleural involvement coupled with the cytologic features of epithelioid GIST can lead to a difficult diagnosis. The patient, an 86-year-old woman underwent surgery 8 years earlier due to a malignant GIST with predominant epithelioid morphology. She now presented with a pleural effusion. Smears showed medium-size cohesive clusters of large tumoral cells with moderate amounts of cytoplasm. Occasional short cords of tumoral cells showing cell windows between them were present. Cells were polygonal, sometimes with eccentric nuclei and no spindle shape. Due to the previous history of GIST, we decided to evaluate immunohistochemical expression of c-kit and DOG1. In both cases, a positive result was obtained. In conclusion, GISTs are rarely responsible for malignant effusions, mainly if extra-abdominal. The few reported cases, including ours, correspond to epithelioid variants that can closely resemble carcinoma. Therefore, the use of immunocytochemistry is extremely important for reaching a precise diagnosis.