The beam from a Linear Accelerator used for external beam radiation therapy is very fine and can be focused and concentrated onto an area that is a fraction of an inch across, without affecting surrounding tissue. If there was any chance of the radiation contacting your genital area, that section of your body would've been covered with a protective lead shield. Your ribs are nowhere near your genitals, so chances are you didn't even need to have the protective shield in place.So, from what you have described, your past radiation treatment would not affect your ability to provide a healthy supply of sperms.