Unfortunately one of the characteristics of radiation treatment is the long-term damage the radiation does to otherwise healthy tissue. In my opinion your incontinence could very well be related to long-term damage that has affected your "INTERNAL" urinary sphincter. This is the sphincter that, along with the bladder neck, work under involuntary control (like your heart muscle) to keep your bladder closed until you can release your urine. Your "EXTERNAL" sphincter is under voluntary control and is the one that stops the flow of urine when it's not appropriate to go (ie. you are stuck in a meeting).
Kegels only affect the external sphincter. If your problem is with the internal sphincter you can do Kegels till the cows come home and you will still likely have incontinence problems. That said, doing Kegels to improve the function of the external sphincter is still a good thing, as I do think you can maximize the capability of the external sphincter, and that will have an overall positive effect.
I too had prostate cancer radiation treatment (in 2011). After several years I developed calcium stones in my prostate & bladder neck that needed to be removed via laser surgery. This procedure left me with a condition called intrinsic sphincter deficiency, which is basically my internal sphincter will not close properly, and I not only have issues with stress incontinence, but also constantly drip urine (like a bad washer on the kitchen faucet). I do Kegels religiously, and they do help overall, but they are not going to help repair my internal sphincter.
Hope this helps!
