Answer:
Well, unfortunatly...hamsters do get tumors very often. it is because they are small animals and cannot take the sickness like larger animals (and us) can. It can start out as a cancer, or grow from not being healthy. Also, a dirty environment can do it. Not being feed well, or having food not meant for hamsters, dirty water can also do it. If you need any help with this, and your hamster...or a friends has a tumor, please contact me! I can help you deal with this, and take care of it yourslef, without dropping tons of money! Also, they can just get sick. And old age can do it.