Patients in the eight-year study who got the more intensive treatment generally survived a year longer.
"A one-year increase in survival in a study like this is very significant," said Dr. Len Lichtenfeld, spokesman for the American Cancer Society. "This is much more than we usually see in cancer studies."
In addition, patients getting the more aggressive treatment were five times more likely to have virtually all traces of cancer eliminated from their blood.
"We have been operating under that assumption for several years now," said Dr. Arnold Rubin, an oncologist and professor at Robert Wood Johnson Medical School in New Brunswick, N.J. The study "confirms what we already know to be the most efficacious treatment."
Patients and doctors have come to believe that when it comes to cancer, more aggressive treatment is better.
But earlier studies of high-dose chemotherapy and stem cell transplants for multiple myeloma were less rigorous and generally inconclusive. And in breast cancer and some other cancers, high-dose chemotherapy has been found no more effective than standard treatment.
Moreover, high-dose chemotherapy can be grueling for patients. The cancer drugs kill healthy cells along with cancerous ones, causing such side effects as severe nausea and hair loss.
Rubin said the new study would help doctors sway insurance companies hesitant to pay for the more aggressive treatment, which can cost up to $150,000, or about three times as much as standard chemotherapy.
In multiple myeloma, a usually fatal cancer that normally strikes people 60 or older, abnormal plasma cells multiply uncontrollably inside the bone marrow.
The study, covering 401 patients age 65 or younger from the United Kingdom and New Zealand, was reported in Thursday's New England Journal of Medicine.
Patients were randomly assigned to receive either the standard or the aggressive treatment. The 200 getting standard chemotherapy received a combination of four cancer drugs at different times over a few weeks, then repeated the cycle every six weeks up to 12 times.
The 201 other patients got at least three cycles of a similar treatment initially, then had their stem cells removed by having their blood pumped through a machine that filtered out those cells.
They remained hospitalized while getting a very high dose of an intravenous cancer drug, which would have destroyed their stem cells and the rest of the blood-forming system in the bone marrow. The stem cells were then reinfused into the patients' bodies and quickly began making new red, white and other blood cells.
The median survival time was 42.3 months for patients receiving standard therapy; that jumped to 54.1 months for those getting the more intensive treatment.
However, because some patients who were failing on the standard treatment switched to the more intense one but were still counted as being in the standard group, the difference may be even more dramatic.
"The improvement in survival is probably greater than one year," Lichtenfeld said.
Forty-four percent of patients in the intensive group saw virtually all traces of cancer eliminated from their blood, compared with 8 percent in the standard treatment group.
Lead researcher Dr. Tony Child, a professor of clinical hematology at the General Infirmary in Leeds, England, said that the aggressive approach proved so effective that it would be nearly impossible to find enough volunteers to take part in another head-to-head study of the two treatments. Participants would have to be willing to receive the standard therapy.