Initially they would look at "total volume" of radiation = total cumulative dose
they have a nomogram of fractions / intensities and would figure out when enough is enough
a delay in active treatment would cause the docs to assess total dose over fractionated delivery interval to see IF they reached their objectives....
remember that radiation ( especially with chemo ) continues to be the "gift which keeps on giving ! " in that the effects of the radiation persist for a while after treatment ends ( and why surgery is not usually done immediately after neoadjuvant chemoradiation ) to let the tissues "recover" a bit and get the maximum effect from it.
Definitely ask the Rad Oncs about this, depending upon your father's exact situation, they may defer / delay / extend / or adjust his delivery fractions to compensate...... i.e. they may extend OR they may quit on schedule ..... only they can tell you that.
BUTT ..... you just knew there had to be at least ONE !!!!
regarding the side effects / discomfort etc.
You should be all over the Oncs and their assistants about that !
There are and can be a number of things which can be done to relieve problems. Let them know and keep BUGGING them until they get your dad a solution.
there are things which can be done before / between treatments to help
there are things which he should not do before treatments .. AND they should have gone over all this with you / him.
ASK questions, communicate problems to them and do NOT stop until you get answers which help your dad !!!
Cheers and best wishes
CRguy