Anytime you are talking about tumors I think your best option is to see your vet. Like you said, your vet may not be able to help if it's too far along, but he/she will be able to help you decide what the best decision is for the health and welfare of your hammie.
It's a little strange, my hamster had a growth on him that we thought was a tumor as well, we planned to take him to the vet, but after about a week it was smaller, and it didn't affect his energy level or anything. It's mostly gone now. We have a friend that had a similar occurrence. I don't know if this is common in hamsters or not, but it may not be as bad as you think. If it affects their disposition and energy level though, it may be best to check with your vet and get their advice.