Hi and thanks for posting

There is some information about side effects of radiotherapy for breast cancer here  

Different units will suggest different creams to use if people do get any side effects. It will depend on how the skin looks so it's sometimes difficult to advise ahead of treatment. 

Once you start treatment , your own radiotherapy unit will be able to talk things through with you regarding what you can or cannot use. Although brand names aren't usually suggested they will be able to say whether any non perfumed soap or wash will be best for you.

There is some general information on looking after your skin during radiotherapy here  

All the best

Wanda