Tumors grow very rapidly in hamsters. I found a tumor in my one-year old dwarf hamster the size of a pea three days ago, and it had doubled in size in one day. She just had the tumor removed today and she is doing fine now. I would suggest getting the tumor removed if money isn't too tight and you really love your hamster - most vets have cheaper pocket pet rates, so the total vet bill might be between $150 & $200, much less than a cat or dog.