Hi, I did not have radiation but had a prostatectomy.
I am a cyclist both indoor and road.
Earlier after the biopsy part, I was back on the bike after a week. I use padded shorts, some shorts are better than others. Also a very slightly padded saddle with a groove, not a cut out. My position puts me on my sit bones and no pressure on my perineum. Experiment, saddles for triathlons have grooves or cutouts in different amounts. Some cycle shops will let you borrow a saddle to see if it suits your profile. Give it a try, good luck.