I had 28 sessions of radiation last year, I also had the SpaceOar inserted prior to the radiation treatment. They will put fiducial markers in at the same time they do the SpaceOar. I would equate the SpaceOar procedure to the prostate biopsy. Actually, I found the biopsy procedure to be more uncomfortable than the SpaceOar. I had almost zero side effects from the radiation. No downtime at all. I was able to exercise , golf and able to carryon with everyday life with almost no side effects. I would say that as far as ED goes, it wasn't a huge issue and still isn't an issue after over a year later. I take viagra as necessary. I can still get an erection without the viagra, but it is better with it. My Radiation Oncologist showed me a study that showed that taking viagra before and during the radiation treatment showed better results with ED than not taking it before
Have your Radiation Oncologist take a look at this study:
https://www.sciencedirect.com/science/article/abs/pii/S0022534714003553
I don't know where you are, but if you take viagra and it is too expensive at your local pharmacy, try Costco. The generic viagra was about 90% less at Costco vs chain drug stores.
PM me if you have any other questions