It's not unusual for a surgeon to save another doctor's life.

But Dr. Colleen Coleman did so by going under the knife to help an ailing colleague who desperately needed a kidney.

Coleman donated to Dr. Brian Dunn, an anesthesiologist she works with at Hoag Hospital Newport Beach whose kidneys failed from chemotherapy he received as a teenager to treat a stomach tumor.

Coleman came through after one donor withdrew her offer and Dunn's doctor advised him against accepting a kidney from a patient with Lou Gehrig's disease.

"I thought, it's not going to happen," Dunn told The Orange County Register.

He had received a kidney from his mother when he was 25, but donated kidneys don't last forever. In late 2015, his health was failing and he could hardly keep pace with his young daughter.

"I started dragging," he said. "Holy crap, I felt bad."

He started dialysis in April and needed to go through the process four times a day to purify his blood. He referred to the time-consuming and tiring process as his prison.

Coleman's intervention also almost didn't happen after an initial screening erroneously said she wasn't a match. Only after the testing company called back in June to say it made a mistake did Coleman learn she was a match.

Her initial eagerness to help out, though, was tempered with second thoughts and fears until she considered Dunn's 6-year-old daughter.

Coleman's grandmother had died of kidney failure when her mother was a 6-year-old girl. "I didn't want his daughter to grow up without a dad," Coleman said.

The surgery last month was a success. Coleman went to see Dunn afterward.

"I wanted to make sure my kidney could pee," she said.

Dunn, 45, said he felt vibrant and grateful three weeks later.

In a note sent to Coleman thanking her for her sacrifice, Dunn credited her with persevering to help him.

"Monday, January 30th is a day I'll remember forever," he wrote. "It's the day that someone did something truly selfless for me. Colleen, you are an answer to prayer and an amazing example to everyone around you."

Coleman, 51, who returned to work to find flowers, a cake and people hailing her as a hero, said she was moved by the note from Dunn.

"I did not understand how impactful it would be to help someone in this way," Coleman said. "There is a benefit to giving. But hero is a very embarrassing word."

In addition to the scars they both bear in their midsections from the surgeries, they also share other reminders of their bond.

Dunn gave Coleman a set of kidney-shaped Tiffany earrings to thank her. She gave him a Tiffany money clip in the shape of a kidney.