Cancer fatigue can be a result of the cancer or its treatment, including radiation. Fatigue is different than just being tired in that sufficient rest does not restore a sense of wellbeing.
Learn about fatigue as a side effect of radiation therapy
Fatigue (Lung Cancer Alliance)
Fatigue in people with cancer (American Cancer Society)
Radiation: what to do when you feel weak or tired (National Cancer Institute)
Understanding radiation therapy: Fatigue (American Cancer Society)