I see that you've expanded your questioning to ask whether continuing to cycle after prostate cancer treating might increase your risk of recurrence. Here's an article that helps to answer that question:
- 8 Facts Cyclist Should Know Biking and the Prostate https://www.byramhealthcare.com/blogs/8-facts-about-biking-and-the-prostate

Bottom line, cycling is good for your health and exercise including cycling can reduce your chances of recurrence. Supporting what @jerryegge said, you may wish to consider what seat you're using for maximum comfort. And above all, continue to be monitored by your doctor.

@tcanavan, did you ask your oncologist about cycling?