I agree with Sephie. The best data I've seen about
secondary cancers associated with treatment for prostate cancer was the Memorial Sloan Kettering study a couple of years ago.
They showed that the ten year risk of secondary pelvic (bladder and rectal) cancers ranged from 2-4% no matter whether the man had radiation or surgery. Furthermore, they found that any increased risk of secondary cancers was entirely explained by age and smoking, and not by whether or not they had radiation.
Others have estimated that odds of radiation-induced cancers for any type of cancer is less than 5 in a thousand. Of course, there will always be the one in a million cases of people who lack the ability to repair radiation damage to healthy tissues, such as occurs with mutations to the ATM gene.
A problem inherent in all long term studies is that by the time we have the data, the treatment has changed. Possibly, highly conformal IMRT, more accurate IGRT techniques, and hypofractionation may have further reduced any risks. Perhaps more importantly, they have reduced urinary and rectal SEs.