Here's a story about Mayo Clinic cardiologist Dr. Mark Callahan that goes a long way toward explaining what kind of person he was. It was passed along by one of the more than 65 friends and coworkers who nominated him as this column's 2011 "Person of the Year."… Unfortunately, this year's Sellnow's Journal "Person of the Year" honor is being awarded posthumously to Dr. Callahan. He was diagnosed late last month with advanced pancreatic cancer. He sent his co-workers a good-bye email less than two weeks ago, notifying them that he didn't expect to recover. He died a few days later, on Wednesday, Dec. 28. He was 59.