My senior cat has lumps and is losing weight. Should I see a vet?

I'm sorry to hear that your cat isn't doing terribly well. There are several different possibilities here. One is that his weight loss is completely unrelated to the masses. As cats get older, they're more likely to develop conditions like hyperthyroidism or kidney disease, which can lead to weight loss. While neither of these conditions can be "cured," both can be managed to some degree with certain medications or diet. The other possibility is, as you mentioned, that these masses could be cancerous. I completely understand not wanting to pursue extensive treatment for cancer, but before making the assumption that it is cancer, I'd try to get some more information. For starters, I think it's perfectly reasonable to take your cat to the vet for an exam and some bloodwork to check his organ function. There's a chance that your vet may find something that's not terribly difficult to manage, but you don't know unless you look. I would expect that your vet would recommend trying to diagnose the masses in order to be thorough, but if you are clear that you don't want to look into them further, it shouldn't be an issue. At the end of the day, you own your cat and are responsible for making choices for him. Your vet's job is to present those choices to you and help you to find the approach that works best for you and your cat. I hope this helps and that everything goes well.