Is getting the x-ray worth it

My dog (Kiki) is an Irish wolfhound cross, 12 years old this year. A couple of weeks ago I noticed her limping and discovered a fairly large hard mass just above her ankle on her hind leg. Without noticing any actual injury, my mind immediately jumped to cancer - old big dog with a lump on a long bone.

Took her to the vet, who suspected the same as me, but with a very outside chance of an abscess. Given the mass is very firm and only slightly warm he didn't think this was very likely. It's also not broken because she can put weight on it and the limp comes and goes.

He suggested an x-ray for diagnosis as they can identify cancer fairly conclusively this way, with the possibility of a bone biopsy if I really wanted to find out exactly what it is (this was not really recommended as it wasn't necessary and a bit more invasive). I booked in for the x-ray

I long ago decided that I wouldn't pursue any treatment if Kiki did get cancer as it's a lot to put her through, and it would be more for me than for her.

I'm therefore starting to wonder if it's even worth doing the x-ray. The vet was pretty confident in his diagnosis given the presentation. Kiki hates the vet and gets very stressed and she would have to be left there all morning, with sedation required for the x-ray. Considering the low chance of learning anything other than what we already suspect, and that if it is what we suspect we won't be doing any treatment other than pain management I'm not sure it's worth the stress to her (and the $600+ expense).

I'm planning on talking to the vet tomorrow, and if there's any real chance it's something treatable I'll happily go ahead with the x-ray. But if not, is there really anything to be gained by having the investigation?