@lizziew I have a rectal spacer.

My "at least high risk" case called for the highest possible dose delivered to my prostate, which was accomplished with direct insertion of a radioactive source right into my gland, that was moved around inside it until all of the gland was roasted, plus, an external beam treatment that added a bit to that while also radiating my pelvic lymph nodes.

I'm guessing that if external beam radiation is limited to the prostate gland, there is less likelihood of intestinal irritation afterwards. Blasting the pelvis seems destined to affect the intestines.

Apparently up to 50% of patients subjected to external beam treatment used to get some sort of bowel problems. The rates are said to be much less with IMRT (intensity modulated RT), and less than that with brachytherapy. The machines are improving all the time.

I get the brachy rate plus the IMRT rate. I'm not that concerned about "acute" problems, as these immediately apparent effects are said to decline fairly rapidly. What happens to a certain number of radiated patients are problems that crop up after many months, or even years later.

It seems clear that side effects are not just due to how accurately the radiation was delivered, or whether there were a lot of treatments or a few.

There has been research into seeing if genetic testing can determine who is going to suffer the most severely from the different types of radiation treatment on offer. The Prostox test is something a person can have done. It hasn't been validated for a combo external beam plus brachy treatment, but it can provide info to patients getting conventional or hypofractionated external beam, or for patients contemplating SBRT.