Would you put quality of life into this equation. Do you think you would give a pet aggressive Cancer treatment? In my mind the pet does not know that the effect of the treatment is to prolong their lives, they just know they feel bad and or sick all the time. Would you ever euthanize a pet, it certainly seems to be bad form to euthanize a person, but it is quite accepted in the animal medical world.
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Not the person you were asking...but a couple of thoughts. With pets, chemo is usually not given as aggressively as with people. Basically if the chemo is making the pet feel bad or sick all the time then they aren't doing it right and they will usually try a milder dosage or milder form of treatment.
To the OP -- it really depends. We've had years that we had few vet expenses, just the annual visit and any needed vaccines. But, this past year and a half has been rough. We did it depending on the situation.
13 year old dog - She had a mammary tumor. We spent about $700 on surgery for it. Chemo is not very effective for this form of cancer. Sometimes the surgery gets it all and it never comes back. Sometimes it does and it becomes harder to treat. We decided to do the surgery once. She tolerated the surgery well and happily lived for almost a year and a half before succumbing (yes, we did euthanize when it was clear it was her time).
11 year old cat - We spent $10k on cancer treatment. This did include chemo. He tolerated the chemo well and went into remission for a time. But, it didn't last as long as would have been expected. And, then he didn't respond to the second line treatment. In his case he had surgery before chemo. It was necessary because of where the tumor was located (it was impeding breathing). He was euthanized when it was clear that the second line chemo wasn't working and when he started to clearly feel bad.
10 year old Saluki dog - This was a shocker. It was totally unexpected. This was a dog that had been healthy and started losing weight very quickly. The big cost was to do an ultrasound with biopsy to try to see what was going on. Essentially while it wasn't entirely clear what was causing the problem, it was a problem in the intestines that wasn't curable. He was euthanized a few days later. We had samples taken to figure out what was going on. It was an uncommon fungus. The cost to diagnose and do everything else was about $1000.
1 year old cat - She had stomatitis. The best treatment for this - even with a young cat - is to remove all teeth. That happened in January and she is doing really well. Cost was about $3300. She has had a dramatic improvement. She had chronic inflammation in her mouth, was constantly bleeding and in pain. Now, she doesn't have any of that. It was so worth it to do the surgery.
The point is that it really depends. For the one cat, treating his cancer made sense as it was a cancer that often responds to chemo. For the dog, her cancer didn't really respond to chemo and surgery was more appropriate and less expensive. For our other dog, we were told that any treatment would be futile so it really didn't make sense to do it.