Bill Bastian was caught completely off-guard when a doctor told him that he had pancreatic cancer and would only have 15 months to live. It was August 2015 and Bastian had thought the pain in his belly was due to something he ate.

“One of the things I thought was, you got the wrong guy,” said Bastian, 69, of Eden Prairie, Minnesota.

His cancer was stage 3, meaning it had not spread from his pancreas to other organs, but had grown into a critical artery just outside his pancreas. That made it inoperable, according to his doctor at the time.

That’s when Bastian decided to seek another opinion at the Mayo Clinic in Rochester, Minnesota. There, he received a totally different prognosis: surgery — and longer survival — were possible.

“If the patients are treated appropriately, and we can predict that their long-term outcome is going to be excellent, [surgery] is worth the risk,” said Dr. Mark Truty, a surgical oncologist at Mayo Clinic.

About a third of pancreatic cancer cases are found at stage 3. Truty estimates about half of his pancreatic cancer patients diagnosed at this stage seek his care after other physicians said their tumors could not be surgically removed. His team’s approach to treating stage 3 pancreatic cancer is different from most other oncology practices.

Historically, doctors have given pancreatic cancer patients chemotherapy or radiation hoping it would cause the tumor to shrink or pull away from the artery or vein it’s ensnared. Truty believes that’s the wrong approach. “You’re going to be sorely disappointed if that’s what you’re expecting,” Truty told NBC News.

The Mayo Clinic approach works like this: Patients are given extended, personalized chemotherapy until levels of a tumor marker in the blood called CA 19-9 fall to a normal range. Then if a PET scan shows the tumor is destroyed, doctors move forward with radiation and surgery.

Bill Bastian was told he'd only live 15 months when he was diagnosed with pancreatic cancer. That was four years ago.

Among 194 pancreatic cancer treated this way at the Mayo Clinic, 89 percent lived longer than the expected 12 to 18 months. The approach has pushed average survival to five years after diagnosis, according to a study by the Mayo Clinic.

And for about a third of patients in the study, Truty said his team “couldn’t even calculate their average survival because they’re alive and doing very well.”

Outside pancreatic cancer experts say the Mayo approach is exciting. “It’s really remarkable to be able to accomplish this,” said Allison Rosenzweig, senior manager of Scientific and Clinical Communications at the Pancreatic Cancer Action Network.

But Rosenzweig cautions the approach won’t work for all patients diagnosed with stage 3 pancreatic cancer. Some won’t respond to the chemotherapy necessary to kill tumor cells. Others are too sick to tolerate the therapy.

Pancreatic cancer received national attention in March when “Jeopardy!” host Alex Trebek revealed he’d been diagnosed with the disease. Trebek, 78, said his cancer is very advanced — stage 4. Stage 4 pancreatic cancer generally means the cancer has spread to other parts of the body. The Mayo Clinic approach is only for patients treated with stage 3 disease.

The American Cancer Society estimates that 56,770 people will be diagnosed with pancreatic cancer in the U.S. this year. More than 45,000 will die of the disease.

Pancreatic cancer is notoriously deadly because it tends to spread quickly, and it’s usually not found until it’s advanced. Rosenzweig explains the pancreas is hidden deep within the abdomen, which makes it difficult to see in imaging. And symptoms are often vague: lower back pain, weight loss, upset stomach.

Bill Bastian said that other than some minor stomach pain, he felt fine when he was diagnosed.

He and his wife, Linda, are looking forward to marking four years post-diagnosis this August.

“I’m feeling very fortunate,” said Bastian. “I feel lucky to be vertical, above ground and moving and doing about everything I did prior.”