The radiation thearapy (brachytherapy) in this case statisticly is every bit as sucessful as any of the surgical techniques. The difference is even if there is a very small chance that a surgical procedure was unsucessful there would always be radiation, or hormone therapy to have as back-up treatments. This may not be possible if a radiation technique is the first treatment. Although it would not entirerly exclude surgery as a fall back option; many surgeons believe that the tissue in and around the treated area would have been too greatly compromised. Then hormone therapy would be the remaining option.
I know swimom has done extensive research in this matter, and I'm sure will soon explain in greater detail what I have left out or missed.