I've survived prostate cancer for 5 years now after radiation treatment, but with the usual side effects. Is it harmful to masterbate?

No, please enjoy. There is no reason not to masturbate, and healthy to have the urge. Is there any chance you might find or have a partner, that might be better.
No. No evidence to say yes. Also, we are assuming the cancer is gone. Just do routine psa checks and you should be alright.
No! Do whatever feels good to you. You are a survivor, you deserve it!