You cannot focus the radiation field perfectly. The whole body will be subjected to (non uniformly distributed) dose of maybe a permille of the reference dose in the irradiation center: there is both leakage from the treatment machine and scattered radiation from within the patients body.
Given that a typical treatment will be about 50Gy (distributed over many fractions), my estimate is for a few ten mGy (mSv) in total to almost every part of the body outside of the direct beam.
(I'm building dosimeters in my day job and work on medical accelerators all the time.)
Given that a typical treatment will be about 50Gy (distributed over many fractions), my estimate is for a few ten mGy (mSv) in total to almost every part of the body outside of the direct beam.
(I'm building dosimeters in my day job and work on medical accelerators all the time.)