How long after radiation should we do a CT Scan?

[MishaW]

My 16-year-old cat has been diagnosed with squamous cell carcinoma in his mouth. He's also got a significant mass in his lung that has not been biopsied - unsure whether it's a different cancer or the same. He's also got a number of small masses in his lungs.

We opted to give him 6 radiation treatments in order to slow the roll of the cancer in his mouth on the theory that we'd rather him die of the stuff in his lungs. In theory, that's less painful.

Other than HATING the ride to and from the vet, he went through radiation including the anesthesia great. A month later, he's still the same easy-going, loving guy and still eating well, which is crucial.

The vet who did the radiation is so excited about how well he's doing, she wants to do chemo too. And I, of course, want to do whatever is best for my buddy. My understanding is that we've potentially just doubled his life span from 3-6 months to 6-12 months. I'm unclear how much more time we think we'd be buying with the chemo. Not much?

I held her off by saying I wanted a CT scan to see how far how fast the lungs are progressing. My question is: what is a reasonable time to do a follow-up CT scan? 1 month? 3 months?

Also, any advice?

Thanks
-Misha