My major factor when confronted with your situation is determining/considering the quality of life and possible future improvements; whether it be a family member or fav pet.
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Iffy very well put. That is exactly what we told clients faced with such hard issues. If the pet might experience some, but not really bad, discomfort while treating the disease and afterwards it would have a good life, we encouraged the client to treat.
Of course there are exceptions. Years ago one of our cats, Fiona, was diagnosed with intestinal cancer. It is treated with chemo and IF the patient goes into remission, their life expectancy is a 1-2 years…with quality of life. Remission was achieved in less than 50%. If they don't go into remission life expectancy is a few months. Cats take chemo really well, nothing like humans. So we considered it until the specialist told us this form of chemo is by injection, not pills, and no, even though I worked in the business, he would not give me the injections to administer myself.
Fiona was a very timid cat who traveled very badly. Open-mouth breathing, wailing, vomit…the works. At her age, she was 17, she had already lived a wonderful, happy life and we simply could not put her through a 45 minute trip twice a week for several weeks with the odds given to us. As it turned out it was the right decision. About a week later she died in her sleep. The specialist told us that meant the cancer was even more advanced than he thought and chemo wouldn't have worked.
On the other hand I remember one of our patients, a boxer whose name escapes me, who was diagnosed with cancer when he was only two. Went through the chemo with ease (dogs also don't have the difficulty humans have with chemo), went into remission and stayed there until he died years later of old age

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Quality of life…that's where it's at.