I had this question with my last dog, a Weimaraner, who had a mast cell tumour on her back leg at the age of 7. They removed it by surgery, but because of the location, couldn't take as much margin as they'd wanted to, so recommended a course of radiotherapy after the op to make sure. I was told that she had a good chance of recovery with the combined treatment; without it, well....
I had to take her to Cambridge vet school (from Kent) every Friday for (I think) about eight weeks for the treatment. She was sedated for the treatment, and though the first couple of treatments didn't seem too bad, the cumulative effects of the course left her quite tired.
However, after the course of radiotherapy was over, she bounced back quite quickly and, apart from a little bald patch on her leg where the radiotherapy had been, you wouldn't have known anything had been wrong with her. She had another five years of very happy and active life after that, until she died at the age of 12, which is a good age for a Weimaraner.
It was expensive (I didn't have insurance) but I never regretted having it done for a minute. We had five more years together that we wouldn't otherwise have had, and she was a happy and active dog who enjoyed life to the end.
If your vet is confident/optimistic that it will give your dog a good quality of life, and if he'd do it for his own dog in the same circumstances, I'd say go for it.
Best wishes to you both 