In my experience the radiation is cumulative, for me the week after treatment ended was by far the worst. I don't know what having a red hot poker shoved up your ass feels like, but bowel movements during that week gave me a pretty good idea!!
My first reaction to the radiation was a bloom of external hemorrhoids They were not at all painful, but they did significantly increase the surface area for burns. The skin burns started to show up at about week three and got progressively worse, by the end of week four some of the burned areas started bleeding.
My tumor was very low, actually attached to the sphincter muscles, so it's likely a greater concentration of radiation had to be directed thru the anal area than if the tumor was higher up. Hope this helps.
Peter, age 65 at dx
DX 4 cm x 4 cm very low rectal adenocarcinoma into the sphincters 01/15
Stage III T3 N1 M0 with two suspicious lymph nodes
26 sessions IMRT radiation with 1,000 mg Xeloda twice per day 03/15 to 04/15
Complete clincal response to the chemoradiation...the tumor shrank completely away 06/15
No surgery...Habr-Gama watch and wait protocol instead
Xelox chemotherapy 07/15-12/15
MRI and rectal exam every three months starting 07/15
MRI and rectal exam every six months starting 07/17