My mum has been offered preventative brain radiotherapy, has anyone had any experience with this and the side effects? 
they have told us it may cause cognitive memory loss  unsure how severe this could be or if there are any other side effects? 

My mum is 77 has a diagnosis of small cell lung cancer, we found it very early and she had the central lobe of her lung removed successfully, she then had a round of chemo and got the all clear for almost a year. The cancer then returned on her back and there is something on her chest, they had a second round of chemo and she is about to start radiotherapy on her chest & back but now doctors have also suggested pri therapy to help prevent the cancer cells spreading to the brain. I am really worried it affects her memory and personality but feel if we don’t do it there is a high chance of the cancer spreading to her brain and that is the one place she has said she doesn’t want it to go to. Does anyone have any advice or more information on this preventive procedure?