Thanks so much for your input Gretchen. At her last vet visit in October we had it drained and the fluid tested. Vet said it was benign, but still recommended we remove it because it looked inflamed. We were torn because it doesn't seem to bother her at all. It went back to it's ping pong ball size shortly after it was drained. Since then it just keeps changing (small for a while, then large...).