In that case I would get multiple opinions from good radiation oncologists - at least three if not more.
Also, if you are a surgical candidate, radiation in the anal/rectal area could also be a factor a surgeon would have to consider since tissue would invariably be affected even with low dose radiation.
All radiation is cumulative, so many low dose sessions are equal in impact to fewer high dose ones.
The initial side effects may be mitigated but longer term they are equal.