Take this time to heal. Things will get better. Being tired is your body's way of telling you to rest. Your whole body deals with the side effects of radiotherapy. Welcome to survivorship. ..you now have plenty more time to get on with things once your body has recovered. A short walk each day might help after the worse is over and you start to recover.
https://www.cancer.org/treatment/treatments-and-side-effects/treatment-types/radiation/coping.htmlhttps://www.bcna.org.au/health-wellbeing/physical-wellbeing/fatigue/