There's good news and bad news. The good news is that since this fish may have Oranda genetics in it, this could actually be a malformed WEN (https://encrypted-tbn0.gstatic.com/i...9PtKuC6aJCGA&s
) so not a tumor. The bad news is unfortunately, there's nothing you can do about this and it appears the fish has had this for quite some time. So unless it bursts, I wouldn't do a thing differently than what you've been doing. The main thing is that you keep the water as clean as possible.
There are a number of different types of tumors that are common to goldfish and thankfully, most of them are benign so nothing to worry about.