Can a man be sexually active during the period that he is undergoing treatments for prostate cancer?

Maybe: Most treatments for prostate cancer have a high chance of causing impotence, so both the desire and ability to undergo sexual activity is compromised. Having said that, i don't think sex itself will compromise the efficacy of treatment unless they involve using drugs that counteract the treatment (e.g. Androgens).

Yes: I assume you mean external beam radiation, the answer is yes for this.

Sex and postate canc: First, sex is natural and good for you and there is no harm in enjoying sex. Potency has libidinal, neurogenic, and vascular components. Sexual ability before cancer treatment is a guide: many have advanced ED before the dx. Each therapy has risk, and the published information is rarely prospective. Ascribing potency failure looking back is biased. If it worked before, likely will work after.