This is a cytospin of an ascites fluid from a patient with widely metastatic adenocarcinoma.
Notice the size of these tumor clumps (see arrows) when compared to the size of the background neutrophils, lymphocytes, and macrophages.
Also, note how close together the nuclei appear in the tumor clump. Think about the separation you would see in a mesothelial clump. These tumor cells are larger than mesothelial cells would normally be. They have considerably larger and more dysplastic-looking nuclei and have much less cytoplasm than mesothelial cells would normally have. These are key differentiating features in the identification of adenocarcinoma tumor clumps in fluids.