The risks of complications from radiation are less than from surgery, but radiation is also generally believed to be less likely to "cure" the cancer (in this case, to kill all the cancer cells) in suitable candidates. Death as a direct result of radiation is possible but very unlikely -- only about 1 in 500 treated men die.
Gastrointestinal problems are quite common during treatment; pain, diarrhea, or rectal urgency may occur in about 33 out of 100 cases. This usually improves when treatment is completed.
Erectile dysfunction (impotence) may result if the nerves responsible for erections are damaged during radiation. These nerves run directly over the surface of the prostate and, depending on the extent and location of the tumor, may be very difficult to avoid. About 25 to 40 out of 100 men are likely to suffer erectile problems after conventional external radiation.
Urinary problems may occur if radiation damages the urethra or the bladder. Mild incontinence may be expected in about 5 to 10 men out of every 100 who undergo prostate irradiation; complete incontinence occurs seldom (less than 1 out of 100). Bladder or rectal injuries are also possible but are uncommon. Urethral strictures may result from scar tissue formation. These cause the urethra to narrow and make urination difficult. About 5 out of every 100 men who go through radiation treatment require at least one procedure to dilate the urethra.
Brachytherapy treatment (inserting tiny radioactive "seeds" in the prostate) can cause difficulty passing urine that can last up to one month after treatment. Long-term issues can include continued difficulty passing urine. Two out of 100 men also experience proctitis, a chronic condition characterized by rectal straining and bleeding. Approximately 10% to 15% of men who receive brachytherapy become impotent.