Salvage radiation and using Cialis during radiation

I understand there was a clinical trial performed that measured the impact that Cialis had on blood supply when taken simultaneously with salvage radiation.  Salvage radiation impeds blood flow, Cialis improves it. I understand the trial was somewhat positive but I can't locate that trial to determine how it applies.

Can you provide a link?  

I was reading Saving Your Sex Life: A Guide for Men with Prostate Cancer by John P Mulhall yesterday and there was some mention of Cialis during radiation therapy having good results for detering later ED. It was not a formal study, however.