Patients with pancreatic cancer that hadn’t spread lived substantially longer when given a combination of four drugs than on a single standard cancer drug — a rare advance for a tough-to-treat disease.

The results, presented in Chicago at an American Society of Clinical Oncology conference, indicate that the powerful chemotherapy treatment known as folfirinox will likely become standard of care for the minority of patients whose pancreatic cancer is diagnosed early enough to be removed by surgery, experts not involved in the study said.

Dr. Richard Schilsky, the group’s chief medical officer, called the research an “immediately practice-changing study” and said it’s the biggest advance for pancreatic cancer in 25 years.

After an average of three years of follow-up, nearly 40 percent of the folfirinox patients were disease-free — compared with about 20 percent who had the standard drug, Gemzar. Overall, almost two-thirds of folfirinox patients were still alive, versus nearly half of Gemzar patients.

Those were unexpectedly good results, said Dr. Thierry Conroy, the lead author and a cancer specialist at the Cancer Institute of Lorraine in Vandoeuvre-les-Nancy, France.

Folfirinox is already standard treatment for patients whose pancreatic cancer has spread.

The outlook has been bleak for patients with pancreatic cancer, an uncommon disease for which there is no screening. About 330,000 cases are diagnosed each year worldwide, including about 55,000 in the United States. About half are diagnosed after the disease has spread; most die within a year of diagnosis. Only about 6 percent survive for five years.

Schilsky said both Folfirinox and Gemzar are “pretty inexpensive” since the drugs are available as generics. Insurance typically covers both for metastatic cancer and gemcitabine for operable cancer, and it’s likely it would cover folfirinox for operable cancer if it becomes standard of care, which several experts say is expected.