I was always told that radiation was a trip to the well that you could make only once considering the high levels of greys that you get in treatment. (There is a maximum lifetime dose issue) But perhaps with the reduced greys that people get through IMRT, that hard and fast rule may be changing. At some point the radiation levels begin to cause their own cancers. You can cause irreparable damage to an area of tissue through excess levels, and it would not surprise me - since the manifestation of that is the creation of non healing wounds - that what you have been told might be right, but I have never heard this before.