Prominent in this case probably means slightly more dilated/larger that expected. As the radiology report said, it is of uncertain clinical significance. As a general rule of thumb, things that are symmetric are usually less concerning. Diagnostic considerations include but are not limited to normal variation versus reflux/increased pressure from a distended bladder. In radiology and medicine in general, when one does not know what something is, a reasonable course of action would be to follow it to make sure it is stable (i.e., unchanged). In all likelihood, I would not lose sleep over this. On future surveillance studies, consider CT urogram, which is a CT tailored to evaluate the kidneys, ureters, and bladder.