A Tale of Two Epithelial Cells

I love this photo because the vet got lucky and picked up two types of epithelial cells in one aspirate!

This was a dog with a large mass protruding from the perianal area which was aspirated...

Which two types of epithelial cells do you see, and how do you know which is the important/neoplastic population?

In this case we see perianal gland epithelial cells on the right (lots of cytoplasm) and those of an anal sac adenocarcinoma on the left (only a little bit of cytoplasm/neuroendocrine appearance).

Because of the presence of the cells on the left, we know this mass is an anal sac adenocarcinoma and are the important population.

Perianal gland epithelial cells, like we see on the right, can be seen in perianal gland adenomas; however, assuming this isn't a dual neoplastic process (which would be extremely rare), we can pretty safely assume that those are normal perianal gland cells that were inadvertently picked up in the process of aspirating the mass.