There are some side effects that are often mild, and only small percent of treated patients have them in more severe intensity and for longer period.
Usually, they disappear soon after the treatment has been completed (lasting for few weeks).
The main side effects are bladder irritation and a need to urinate more often.
Rectal irritation or discomfort with diarrhea and bleeding is another possible side effect.
Some patients become impotent as a result of this therapy, and this problem tends to develop gradually over 6–12 months.
Localized burning of the skin similar to severe sunburn, bowel impaired function and fatigue can be experienced, too.
Ask your oncologist about conformal radiotherapy (CFRT), or intensity-modulated radiotherapy (IMRT), both new techniques with more accurate targeting of the cancer and a lower risk of side effects.

Best wishes!