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 Sunday, 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.

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 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 John Niederhuber, director of the National Cancer Institute.

It's way too soon to declare victory. No one knows how long the benefits will last, whether people will need "boosters" to keep their disease in check, or whether vaccines will ever be a cure. Many vaccines must be custom-made for each patient. How practical will that be, and what will it cost?

Those are all good questions -- but there are no answers yet, said Richard Schilsky, a University of Chicago cancer specialist who is president of the American Society of Clinical Oncology.

Several vaccine studies were reported over the weekend at the oncology group's annual meeting in Florida.

A big problem has been getting the immune system to "see" cancer as a threat, said Patrick Hwu, melanoma chief at the University of Texas M.D. Anderson Cancer Center. Viruses like the flu or polio 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. "It's a mimic to what you're trying to kill, a training device to train the immune system to kill something," Hwu explained.

To make the attack as strong as possible, doctors add a substance to put the immune system on high alert.

Researchers gave 41 patients the shellfish protein and an immune booster; 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.

Big gains also were seen with a neuroblastoma vaccine developed by the cancer institute. In a study of 226 patients, 86 percent of vaccine recipients were still alive after two years versus 75 percent of others not given the vaccine.