Before heading home to Maine to spend the holidays with his family, PATRICK DEMPSEY sat down with ET's JANN CARL to talk about Breakaway from Cancer, an organization that raises funds to support people living with cancer, for which he is filming a public service announcement.

It was the "Grey's Anatomy" doc's personal experience that led him to become involved with the organization. His mother, AMANDA, is a two-time survivor of ovarian cancer.

"My mom's a very private person, so talking about it is always very difficult, and I respect her privacy about it," he tells Jann. "This is something to honor her a little bit."

As the father of twin boys, Patrick also addressed the medical issue that DENNIS QUAID's two babies went through: "There's nothing worse than hearing your child is going to go through something like that. The person that administered that shot has got to feel horrible. Medical science is not perfect and that's a real tragedy. Hopefully, they will recover, but only time will tell."

As mentioned, Patrick, wife JILL, daughter TALLULAH and sons DARBY and SULLIVAN will be spending a white Christmas in New England.

"It's the first Christmas with the boys. They'll be walking by then, so we're excited," he says.

As a result, he won't be attending his "Grey's" co-star KATHERINE HEIGL's wedding, but he shares, "I am so happy for her. I've never seen her so happy. He's a great guy, too, and they are a great couple."