Going on a liquid "cleansing diet" at a point in time when everyone has difficulty maintaining proper nutrition, caloric intake of any consequence which compromises healing, and keeping up hydration, seems nothing less than counterintuitive to me. This is a point in time when you want to be calorie loading. As to doing it in the last three weeks of treatment, you are likely going to be on a specific liquid and semi solid diet dictated by a nutritionist, not on a cleansing diet. The idea sounds like something that will actually slow your healing processes, at a time when you need them the most.

Anything that you are thinking about doing today is likely going to be not on your mind after about three weeks of radiation when most people hit the wall. Nevertheless, anything that you decide you want to try you should get signed off on by your treating physicians before committing yourself to.