External Beam Radiation
Each treatment lasts only a few minutes. Men usually have five treatments per week in an outpatient center over a period of seven or eight weeks. The treatment itself is painless. Newer forms of this type of treatment appear to have a good success rate with fewer side effects.
Side effects can include diarrhea with or without blood in the stool, rectal leakage, and irritated intestines. Sometimes, normal bowel function does not return after treatment is stopped. Both during and after treatment, other side effects might include frequent urination, feeling like you have to urinate all the time, burning while urinating, and blood in the urine. Also, external radiation therapy can cause tiredness that may not go away until a month or two after treatment stops.
About 30% to 60% of men become impotent within two years of having external beam radiation therapy. Impotence usually does not begin right after treatment (as it often does with surgery) but develops slowly over one or more years. Impotence can often be treated, for example with Viagra.
Source: American Cancer Society