I also have a grey gelding with melanomas, much like your mare’s, as well as his sheath, tailbone, head. 
No vet that has seen him has suggested treatment/surgery. I think they just haven’t seen any helpful results with Oncept (maybe because it was already too far along when I got him) and the surgery location is clearly not easy (if at all) to manage. So what I do is keep an eye on him and hope for the best. He’s 17 now, and I know he won’t be that 27 year old retiree ever. I honestly worry more about the potential for internal ones, that I cannot see. And whenever he does something weird, I immediately worry. But, he lives at home with me, so I know him really well, know all his pooping habits and I’ll know when things have turned a corner and I’m prepared to let him go when that happens. I’d say over the past year, he might have something started brewing internally that is causing a bit more difficulty than before, so again, I’m just on watch. At this point, if he had an impaction for some reason, a very small hand would be required for any type of rectal exam, and we might be at that point where it can’t be done. Best wishes to you and your mare.
I’m sorry this is happening to your mare. At 21 yrs, I would be hesitant to have this type of surgery done in that area.
I agree that an in person consultation is needed and an effort made to do what is possible to see what internal melanomas are present.
Keep in mind that surgeons, both veterinary and human, are trained to perform surgery and think in the manner of “what can we do surgically?”. Sometimes they aren’t the best arbiters of can-it-be-done vs should-it-be-done.
I hope you can get several opinions (not just from surgeons), see if you can find out if she has any internal melanoma issues as well, and then you make the call.
With a 21 year old mare I’d be unlikely to put her through the ordeal of surgical excision and recovery. It’s definitely a difficult call that you have to make.
Best of luck, whatever you choose to do. Hugs to you, from Colorado.
I have a horse that’s been on Oncept for almost 11 years. We started him on it when his melanomas were relatively small. It has done a good job of slowing the growth of his melanomas. He did have to have one round of laser surgery on his penis but otherwise he is generally healthy.
Ohio State (who manages him) has used Oncept on more than 50 horses. It works in the best of circumstances on about 50% of the horses and seems to work best before the melanomas grow aggressively.
Oncept does not have to be administered by a board certified oncologist or internal medicine vet. Our local vet administered it before he retired and I know other regular equine practitioners who administer it. It can be hard to obtain - the supply seems to vary considerably. It has been hard to come by the last few months.
I am so sorry about your horse - I hope you and your vets can arrive at an approach that keeps her comfortable for as long as possible.
I have a gray stallion, will be 23 in a month. He came to me at 12 with some melanomas - a couple in the mane, some in his sheath and some perianal. Fortunately for me, they have changed little in all these years. I’m no expert on the surgery part by any means, but I would personally be wary of the after care, healing process, risk of infection and discomfort. Also, note re the laser - it actually creates burns and I can tell you from personal experience in my “nether region” that there is significant discomfort, and some itching. If I were a horse, I’d be rubbing on a fence… Much as I love my horse, and want him to live forever, I dont know that I would put him through the removal.
I do think that it might be worth a second opinion; perhaps your vet could consult with maybe New Bolton or other Vet Hospital and see if they agree w/ Cornell or have other opinions. Good luck to you; these decisions are complicated…
The vaccine can only be purchased by board certified veterinary oncologists or internists (large or small animal). General practice veterinarians sometimes have channels where they can get the vaccine (ie. they buy it from a specialist), but that can be hairy legally depending on the state.
Wishing you all the best!
Do you have any info about Oncept’s clinical trials? It seems to me that they started the trials before COVID and I haven’t heard a word since. Neither has OSU. It sure would be nice if it were approved for horses so there wouldn’t be all these off label workarounds.
I am not certain why Oncept has traditionally been not only hard to obtain but also hard to get consistent info from the manufacturer. I have been lucky that OSU has generally been able to pry some loose when I need it, but it seems like the manufacturer would ramp up for the current demand. Mystifying!
There are not any current clinical trials that I’m aware of.
There were manufacturing problems earlier this year, so it was on backorder for a long time. They truly just could not make enough.
Dr. Philips (the Oncept for horses pioneer) was supposedly leading the trial to make equine use on label. He was scheduled to present at AAEP one year (possibly pre COVID?) and then canceled because the trial was starting. I (and my contact at OSU) haven’t heard a word since. Just wondered if there was news anywhere that we had missed. If I get really bored some time maybe I’ll call Boehringer and see what I can turn up. I might be a retired special librarian but I ain’t lost the touch for a deep dive! 
I had a gray mare w same. Looked just like your photo. They were also along her tail. I did the surgery, the recovery was messy but doable. Samples of the tumor were sent to Cornell, I think (it was 6 yrs ago) and they made a vaccine from the tumor sample. She got the vaccine and for 6 mos, the tail tumors shrunk ( they were unable to surgically treat them) but after that, they came roaring back. She deteriorated after that and the assumption was that melanoma had spread internally and I had to euthanize her. Her tumors were unusually aggressive. Of the 20 horses that leesburg had treated w the vaccine, only 2 didn’t do well, and they were both PREs. I was told that without surgery, she’d get to the point where she was unable to defecate and then would need to be euthanized.
