Dr. Phil Mitrano is the court-assigned therapist of prolific pedophile Roy Barnett. Mitrano didn't strain trying to psychoanalyze or treat Barnett, as he knew almost instantly Barnett had no hope of rehabilitating.
When Dr. Mitrano was interviewed by Manhattan SVU, he wasn't surprised Barnett reoffended and molested Sam Cavanaugh. He knew Barnett was a rare pedophile with an uncanny knack for taking time to find boys to sexually abuse even as strangers, with so proud and psychopathic an unwillingness to reform, Barnett boasted to Dr. Mitrano about having molested one hundred boys.
After this and additional interviews, it was plenty to catch Barnett with enough evidence to reconvict him from tapes of the molestations, and fighting to have it accepted in court desite them being obtained from Sam without a warrant. (SVU: "Guilt")