ORLANDO, Fla. — First there was surgery, then chemotherapy and radiation. Now, doctors have overcome 30 years of false starts and found success with a fourth way to fight cancer: using the body's natural defender, the immune system.

The approach is called a cancer vaccine, although it treats the disease rather than prevents it.

At a cancer conference yesterday, researchers said one such vaccine kept a common form of lymphoma from worsening for more than a year. That's huge in this field, where progress is glacial and success with a new treatment is often measured in weeks or even days. Several vaccine studies were reported over the weekend at the American Society of Clinical Oncology's annual meeting.

Experimental vaccines against three other cancers — prostate, the deadly skin disease melanoma and an often fatal childhood tumor called neuroblastoma — also gave positive results in late-stage testing in recent weeks, after decades of struggles in the lab.

"I don't know what we did differently to make the breakthrough," said Dr. Len Lichtenfeld of the American Cancer Society.

Instead of a single "A-ha!" moment, there have been many "ah, so" discoveries about the immune system that now seem to be paying off, said Dr. John Niederhuber, director of the National Cancer Institute.

Still, it's way too soon to declare victory. No one knows how long the benefits will last, whether people will need "boosters" or whether vaccines will ever be a cure.

A big problem has been getting the immune system to "see" cancer as a threat, said Dr. Patrick Hwu, melanoma chief at the University of Texas M.D. Anderson Cancer Center.

Viruses like the flu are easily spotted by the immune system because they look different from human cells. "But cancer comes from our own cells. And so it's more like guerrilla warfare — the immune system has trouble distinguishing the normal cells from the cancer cells," he said.

To help it do that, many cancer vaccines take a substance from a cancer cell's surface and attach it to something the immune system already recognizes as foreign — in the lymphoma vaccine's case, a shellfish protein. To make the attack as strong as possible, doctors add a substance to put the immune system on high alert.

In the lymphoma study, doctors gave 41 patients immune boosters; 76 other patients were given those plus the vaccine. After nearly five years of follow-up, the average time until the cancer worsened was 44 months in the vaccine group and 30 months in the others.

Doctors not associated with the vaccine studies are cautiously optimistic.

"We've raised so many false hopes in the past," said Lichtenfeld of the Cancer Society. "What's different this time is we have the science reports to back up improvements."