Do you stop treatment for a prostate cancer patient diagnosed with acute radiation proctitis with significant pain?

I would first check the treatment fields and treatment setup to ensure that there isn't excessive dose going to the anorectum.

As with all toxicities in radiation oncology, one must exclude other possible reasons for the anorectal pain such as hemorrhoids, infection, or inflammatory bowel disease.

I...

I agree with everything above from @Dr. First Last. As an addition, start with an external anorectal exam before referring for endoscopy. I had a patient several years ago who presented with severe "rectal" pain and hematochezia when passing stool that started during the first week of standard fract...

It would be very unusual. No. I would check the dosimetry and medical history. That said, unlike head and neck, a day or two off is not a big deal.