How do you manage early rectal symptoms such as tenesmus in a patient undergoing pelvic radiation for prostate cancer?

If patients are having symptoms out of proportion to what you would expect, I consider other potential causes, especially if the usual management approaches aren't working. I've had a few patients over the years with undiagnosed Celiac disease, who got severe early diarrhea during a course of external beam for prostate cancer. Once the diagnosis was made, the management became much easier. For early diarrhea, I have had good success with diet modifications. Low fiber diets work in my experience and it's worth giving patients these instructions at the start of treatment. If necessary, the diet changes can be supplemented by antidiarrheals.

Tenesmus seems to be more related to an inflammatory process in the anus, so I would check your fields ...