Melatonin is generally associated with supporting the body’s natural sleep cycle.  In addition, recent studies show that melatonin has both anti-tumor properties and can protect against healthy tissue damage during radiation treatment.  Various doses have been studied, all yielding positive benefits.  

* Please note that in some people, increased doses of melatonin can cause vivid dreaming and grogginess upon waking. Therefore, it's best to start with a lower dose and build your tolerance to higher dose over time.
 
Reference:
Can Melatonin Help Us in Radiation Oncology Treatments?
 
The melatonin immunomodulatory actions in radiotherapy
 
The radioprotective effect of melatonin against radiation-induced DNA double-strand breaks in radiology