Okay, then the ED is more likely to be as a result of the radiotherapy, hormone treatment can cause loss of libido but doesn't usually cause ED. ask for a referral to an ED nurse or andrology clinic if there is one in your area. They will be able advise you on things like bigger 'event' doses of cialis (rather than the daily dose you are currently taking) or perhaps trying Levitra (a different tablet), muse (pellets) or injections.
Did you ever get the vacuum pump?