The vet didn't say what type of tumours they were because I don't think she knows for sure. The didn't seem to be attached to any organ from what I understood, so the tissue testing will have to be done to give us a better idea of what's going on. She did aspirate the tumour in her throat in February and saw a lot of lymphocytes, indicating it was malignant. That tumour and the other two like it were similar to other mammary tumours I have had in the past, although I have never had a mammary tumour that grew internally like the one near her spleen or one known to be malignant. If they are malignant mammary tumours, they are not the same type as her first lump. We had that one tested when it was removed (I thought it was unnecessary at the time but it's proving to be helpful in interpreting the results now) and it was your typical benign mammary adenoma.
I think most or all vets do necropsies. Some send theirs out to pathologists though, which shrinks the window of time you have to get the body to the vet. (As an aside, I knew a while ago I wanted one done so I got the estimates and asked about what to do if she passed at home over the last month.) It can be pretty expensive though, otherwise I would get it done more often. My vet charges $135 for the necropsy (done at the clinic, at the macro level), $92 for histopathology (infected tissue is examined what bacteria and viruses (not sure about the virus part - there a so many it's hard to test for them all) are present and possibly give an idea of effective treatments), and $146 for tissue testing (I can't remember the technical name for this - examines the tumours, organs, etc.). The last two are done at outside labs. I left it up to the vet whether to get the histopathology done and I don't think she is sending anything out for that.
I'm very glad I did it though, although I doubt I will learn much in how to prevent something like this in the future, I am more confident in the efficacy of the implants (I trust my vet and his faith in them, but it's better to actually see the proof) and if I ever have a necrotic tumour I will push for antibiotics at the first sign of infection. (In this case, paralysis was a sign of infection.)