I found this article, don't know how helpful it is. So sorry to hear it wasn't a simple bee sting. I'm not sure if I would have any surgery done, it seems when invaded cancer has a way of fighting back in some instances by growing more. Best of luck to you and my prayers are with you
Fortunately, some of the most common types of cancer in the horse are benign; they are non-aggressive and do not spread to other parts of the body.
Grey horses frequently have nodular masses or "melanomas" about their body. These are most frequently found under the tail and around the dock, but can also occur behind the jaw and in the eye. With the aid of instruments such as laparoscopes and endoscopes to look inside the horse, vets can often observe small melanomas within the abdomen of
greys and in the guttural pouches (throat area).
In many cases, melanomas remain small and harmless to the horse. This is contrary to the situation in humans, where a large number of melanoma cases are very serious.
However, on rare occasions something that starts as a small, innocuous black lump under the tail or below the ear will spread throughout the body and transform into an invasive, destructive cancer.
It is for this reason that vets often note the presence of melanomas in grey horses presented for pre-purchase examination and they may be a cause for concern. It can be almost impossible to predict how they will progress on one clinical examination alone.
It has been estimated that 80% of grey horses, of either gender, aged more than 15 develop melanoma, some of which will behave aggressively and not be amenable to treatment.
Many of the cancers that occur in humans are now amenable to treatment through surgery, chemotherapy and radiotherapy, and these techniques have been applied to equine cancer with considerable success.
The treatment of particular cancers depends upon the type involved. Some types of cancer cell are very effectively killed by radiation, while others are relatively unaffected.
It also depends upon the location of the cancer. A cancer that is confined to the skin can have creams applied; those deep within the chest or the abdomen are relatively inaccessible.