Among them are more than 12,000 Floridians who have signed up for a health survey.
"This is statistically unheard of," said Tallahassee resident Mike Partain, 41, a breast cancer survivor who was born at the base and is looking for others like himself. "We've got a cancer cluster that defies explanation."
The cluster is expected to be discussed Thursday when the U.S. Senate Committee on Veterans Affairs holds a hearing on contamination at U.S. military installations. A Marine Corps major general is expected to testify, as will Partain.
In addition, Sen. Kay Hagan, D-N.C., told the St. Petersburg Times on Tuesday that she is working to get the Senate Armed Services Committee to call a hearing on water contamination at Camp Lejeune.
"I'm not a scientist or a physician," Hagan said of the breast cancer numbers. "But that seems like a significant number to me."
The Times first reported in June that Partain had found nine other men with breast cancer and ties to Camp Lejeune. After that initial story, eight others stepped forward. The total climbed to 40 after CNN broadcast a report about the cancer cluster last month.
A man has just a 1-in-1,000 lifetime chance of getting male breast cancer. Only 1,900 a year are diagnosed with the disease.
"This definitely raises a few eyebrows," said Dr. John Kiluk, a breast cancer specialist at the H. Lee Moffitt Cancer Center in Tampa. But Kiluk also noted it can be difficult to link a disease to environmental exposure.
He and other scientists agree more study is needed. Activists want a full epidemiology study.
"That does sound like a much higher number of male breast cancer cases than you would expect," said Dr. Ann Aschengrau, an epidemiologist at the Boston University School of Public Health who has called for a federal health study of Lejeune residents.
Aschengrau said she would expect no more than two male breast cancer cases in a population of 100,000 people. So if a million people were exposed, 20 cases might be expected, she said.
A Marine Corps spokesman declined to comment on the cancer cluster, saying epidemiologists were better qualified. But the spokesman noted the Marine Corps had spent $14.5 million on research initiatives regarding health issues.