Not sure that comparing x-rays to fish oil is a good comparison of risk/reward.

People (like myself) take fish oil for a variety of reasons, in my case to lower the risks of serious heart disease over the risks it might contribute to prostate cancer.

There are even studies that show fish oil could reduce the chances of prostate cancer: http://www.ncbi.nlm.nih.gov/pubmed/12540506

Ultimately each person has to judge the benefits/risks for themselves until better data is available. It's also worth noting that prostate cancer is increasingly a risk for all men as they age, fish oil or not.