Oh @DanFmDorking sorry to see you back here
I'm not sure I'll be much help on the radiotherapy front, I had 30 sessions across 6 weeks (5 days a week) plus weekly chemotherapy so I have no idea which caused which side effects.
My radiotherapy was predominantly my neck, on account of head and neck cancer!
Obvious immediate things:
Burns - honestly horrific, with any luck yours won't be as intensive but I was given a gel to use and I would say use it religiously. Even if the worst happens and like me the skin blisters and peels off, it does heal relatively quickly. Think of it like extreme sunburn. I found a silk scarf useful, anything touching or rubbing near the area was uncomfortable, loose or low cut t shirts, shawls so I could slouch it away from the area etc
In 2 days it'll be a year since treatment stopped and I now I use factor 50 suncream everyday, even cloudy windy days like this weekend.
Pending where the radiotherapy is/ burns - natural materials as well, cotton, silk and let it breathe as best you can.
Longer term:
Again, this is likely because of the area for me, but I have to use a high fluoride toothpaste and mouthwash as well as nightly mouth guards to soak my teeth and prevent bone decay.
It also severely damaged my saliva glands, so dry mouth and I drink like a fish...
Extreme fatigue and brain fog has been a killer, thanks to the chemo we now play "cancer or menopause" every time I get a hot flush, forget things or can't sleep I tell myself it's just perimenopause NOT anything sinister, I'm guessing that's a game you won't need to play