I have a dwarf hamster. At 2+ years he developed large liquid-filled lumps on this chest (both sides) and later on his rump. I took him to a vet who told me that these were cysts and if he was younger, they could drain them, but at his age, it was best to leave them be, as long as he was happy and otherwise healthy.
Is your hamster's lump feel like it is filled with liquid? Or is it hard?
This happened to my daughter's dwarf hamster many years ago. I took it to a special small animal vet and found it was a mammary tumor. Due to their short life expectancy, it wasn't worth spending the money for surgery. Especially when there is a good chance the hamster will die during surgery as well. I did the only humane thing I could think of and had the little thing put to sleep.
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