I just have to pop in and give my 2 cents..
Gavin had a bump biopsied (real tiny bump on his 'ankle' that was only found cause it made his fur puff out) and it was sarcoma...that one needed radiation because invasiveness and location prevented complete removal.
....I found another bump that felt like a big bite..that was a mast cell tumor. This one was on his rump and was really really hard to find cause the hair is coarser there. Hard to find as in when I had the vet in to check both his bumps I couldn't find it and thought it had resolved (this meant 2 surgeries instead of 1, which I still beat myself up about). And when we had it taken out, it took me and the vet tec 5 minutes to find it!
thankfully the mast cell tumor was removed with clean margins so no further treatment.
Check all those bumps!