The numbers are tiny, and experts don't know whether there is any link between the illnesses and toxins released during the disaster.
But doctors who coordinated the study, published Monday in the Journal of Occupational and Environmental Medicine, said people who worked at the site should continue to have their health monitored.
"What we are trying to get out there is: Be alert," said Dr. Jacqueline M. Moline, director of the World Trade Center Medical Monitoring and Treatment Program at the Mount Sinai School of Medicine.
The researchers looked at 28,252 emergency responders who spent time amid ground zero dust and found eight cases of multiple myeloma.
Those findings were no surprise. Multiple myeloma is the second most common hematological cancer in the U.S. after non-Hodgkin's lymphoma. Normally, researchers would expect to find about seven cases in a group as large as the one examined in the study.
However, four of the people who fell ill were under age 45, and multiple myeloma is thought to be more rare among people of that age. Under normal circumstances, researchers would have expected to find only one case of the disease in that age group.
Those four young multiple myeloma patients included one officer who was caught in the dust cloud on 9/11 and then spent months working long hours at the site. Another spent 111 days at the Staten Island landfill where the rubble was sifted. Two others had less exposure, working 12 and 14 days each in the pit and rubble pile.
The study said it is possible the monitoring program was simply more effective at finding the illness among people who wouldn't ordinarily be subjected to intense medical tracking.
Nevertheless, Moline said, "You shouldn't be seeing so many cases of myeloma in younger folks." The median age of diagnosis for that cancer in the general public is 71.
Several groups are studying New Yorkers exposed to toxic dust when the skyscrapers collapsed.
To date, no study, including the one published Monday, has established a link between that dust and cancer, said Lorna Thorpe, a deputy commissioner and epidemiologist at New York City's health department.
The timing of the four cases examined by the team at Mount Sinai also raised questions about whether they are related to their work at ground zero, she said.
Most research on multiple myeloma indicates that it usually takes 10 to 20 years for someone to develop that cancer after an environmental exposure to a carcinogen.
In these cases, the cancers were diagnosed in as little as three to four years after the attacks, suggesting that something else caused the disease.