Sorry to hear about your mare's turmors. Is your mare mostly grey or have large parts of her body that are grey with pink skin? You can see this best when wet? Sarcoids are the first thing that comes to mind. They most often first appear around the ears, muzzle, under the tail, anus and high between the back legs. Most often, of course there are always exceptions. These slow growing tumors are connective tissue based and can also be internal. If they are on parts of the intestines they can cause blockages and colic. Sarcoids are often raised, but can also be flat. They bleed easly and often return if removed. Procedures that boost the ammune system are often helpfull. Tagamet has also been used with some success in getting the tumors to reduce in size. Drugs that help the bodies natural immune system may be of some help. HERDA is most often seen in horses that are younger, 3yrs or so.
Are these growths under the skin, easy to move, irregular in shape or more round?
Your states Vet University may be a great place to start. They are often on top of the latest treatments, often fees are very reasonable, care is most often excellent and you get the top minds. There are many things that can cause growths or tumors, many types of tumors and many different treatments. Everyday advances are made in medicine. Schools are often more open to trial's and non-convential treatment. Anything your horse is able to teach teachers and students will benefit the horse community. I wish you luck? Does your horse sweat normally? Good Luck