I'm sorry to hear of your post-brachytherapy problems. My husband had treatment last year (where does the time go?!) and never had any problems from the get go other than some fatigue. The frequency will (or should) slow down - did your doctor prescribe you any Flo-Max - that seemed to help my husband tremendously.
I don't know where you heard Viagra causes a return of tumours - our Brachytherapist recommends one of Viagara, Levitra and/or Cialis so I cannot imagine her recommending something that would ultimately become a concern.
Best of luck to you - if you have any more questions please post.