Tumors are very common in rodents so you need to have it checked out as soon as possible in case it is malignant. Some vets do not work on rodents so you need to call around to your local small animal vets and see which ones do. It needs to be looked at soon for several reasons:
If it is malignant it needs to be removed.
The smaller it is the better as it is easier to remove, it may not have spread and the cost will be less to have it done.
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