BOSTON -- Boston hospitals are celebrating a medical milestone as doctors at the Dana Farber Cancer Institute, along with Brigham and Women's and Children's Hospital announced Wednesday their 1,000th stem cell transplants on a donor and recipient who are not related.

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NewsCenter 5's Rhonda Mann reported that Stephen Collins developed Leukemia and needed a lifesaving stem cell transplant, the odds were actually in his favor he would find a match. But when that one failed and he needed a second transplant, the accounting professor knew the numbers didn't stack up as well.

"Some people don't find donors, so it's not a guarantee by any means," Collins said.

Stem cell transplants -- most traditionally done through bone marrow -- have been performed for about 30 years. In a sense, it's a way of replenishing a sick immune system.

"By basically erasing the underlying disease and replacing it with new, healthy stem cells, we can completely regenerate a new healthy blood system," Brigham and Women's Hospital Dr. Joseph Antin said.

When a stem cell transplant is needed, doctors first look towards a family member -- in hope of finding the best match possible. But 70 percent of the time, they are forced to look for an unmatched donor.

That's easier now than ever. Doctors can tap into a registry of 9 million donors worldwide to look for a match, and advances in techniques and medicines have also helped save many more lives.

"Improvement in outcome has been dramatic. Improvement in supportive care, in typing, in antibiotics, in transplant rejection medications," Antin said.

Collins did find another match and was transplanted again last month. That makes him the 1,000th Dana Farber patient to receive his stem cells from an unrelated donor.

"It wouldn't matter what the number was. I'm very grateful for the treatment I've gotten here and the care. It was a great thing to have in my backyard," he said.