Orbea, if you have having normal radiotherapy there is no reason to avoid cycling before or during your treatment. If, as the treatment progresses, you experience urinary hesitation / retention due to the prostate swelling, you can stop cycling until it settles down.
If you are having brachytherapy or the spacer gel, best to avoid cycling for a while afterwards.
Regardless of treatment, you can help yourself by buying a prostate friendly saddle - available from most well-known stores!