According to information that was just posted in a link in the General Board, "Good Website",
"Repeated Irradiation
A previously held dogma that normal tissues can only tolerate a specific maximal lifetime dose of radiation has been challenged by studies in which second doses of radiation of up to twice the expected tolerance of normal tissue have been given with a surprisingly low incidence of serious toxic effects.82,83 These clinical data support laboratory investigations that have demonstrated that normal tissue may recover in time, suggesting that it may be possible to give additional doses of radiation in many types of tissue.84 This approach is currently under investigation by multiple clinical-trial groups.85"