Never worry about offending your doctor when they tell you they have no idea what is causing the pain and is not being proactive enough in discovery or resolution of such.
Some men do have groin pain with their back issues, but the fact you have/had prostate cancer is a natural identifier with your pain there.
My son was kicked in the the testicles as a young child by another child. As a teenager, he found a lump on one of his testicles because he was having pain there. The doctor said it was due to some trauma he had endured and that for him, removal of the lump was about all they could do to resolve this problem. Because of his age, the doctor recommended that he leave it be and just monitor it, and then if it becomes an constant irritant, then to maybe discuss surgery at that time. He wants children, so he's decided to watch, monitor and deal with any flare ups. He's been fine for awhile now, so hopefully it won't cause him too much trouble moving forward.
I am with Carol.. go see another urologist. You need several opininons on this important issue. Take care and let us know how you are doing. Tammy