I really doubt she can point to any evidence supporting that claim, but I'm certainly not going to tell you to ignore your oncologist.
I had same comment from radiotherapy when I was starting my treatment, but my oncologist told me it was rubbish, and cycling (or any exercise which increases blood flow in the prostate) during radiotherapy probably improved the effectiveness of the radiotherapy. Radiotherapy works best on cancer cells which are well oxygenated.
I switched to using a noseless saddle like the one below during my radiotherapy, and I've stayed with it ever since. It's not so much being noseless, as that it has absolutely nothing under the perineum (not just a little dent like many saddles do, but a hole right through. Actually, it has very little except under your sit bones (pelvis), so you can't sit on any part of your anatomy you shouldn't, unlike most male saddles. It does take a few rides to adjust it comfortably and get used to it. The lack of nose means you need to use your arm and leg muscles to keep the saddle under your bum and not slide out sideways.
https://www.amazon.co.uk/gp/product/B07R61NZ4V/ (no longer available, but you will be able to find another similar one.)