Mammary tissue can be found in places you might not expect! Tumors start as a small lump and grow into larger lumps, some lumpy and some smooth. Generally the tumors that feel like smooth balls of fat under the surface, and which are not tightly attached to the underlying tissue, are the least serious and are easiest to remove.
True tumors will not go away on their own, and there is at present no real ‘cure’ except surgery, although you can do other things to improve the quality of life – see steroid treatment below. Your vet may be able to operate to remove a tumor, but bear in mind that a secondary tumor often grows within a few months of surgery.
If you decide not to operate, mice can live very happily even with large tumors for some months. If the tumor starts to ulcerate it will become sore and raw on its surface; at this stage, either removal or euthanization is necessary. Steroid treatment can make the mouse’s last few months more comfortable.