Five of the first six actors listed in the cast of The Conqueror, at the Internet Movie Database were diagnosed with cancer.
The probable cause of the cancer epidemic, which resulted in 91 of the 220 cast and crew members, was that eleven U.S. atom bomb tests had taken place in Yucca Flats, Nevada in 1953.
After the testing, the radioactive fallout drifted to the site close to St. George, Utah where the movie was shot, exposing the cast and crew to the radioactivity, for 13 weeks.
To further strengthen the thought, that the radioactive fallout caused the cancer, half the citizens of St. George had contacted cancer during the next thirty years.
46 of the movie cast and crew that had contacted cancer died from various forms of cancer.
Dr. Robert Pendleton, professor of biology at the University of Utah, stated that 30 people, diagnosed with cancer out of a group of 220, would be closer to the normal number, of cases for that many people.
He also thinks, because of the tie-in of cancer with the movie, that anyone wishing to file suit would have a good case in a courtroom.
John Wayne, Susan Hayward, Agnes Moorehead, John Hoyt and Pedro Armendariz all died with cancer. Armendariz would be diagnosed with cancer four years after movie was completed and shot himself when he found out his case was terminal.
Smokers among the victims, including John Wayne and Agnes Moorehead were heavy smokers which made them even more susceptible to cancer from the radioactivity.
We may never be 100 percent sure, that the radioactive fallout caused the cancer of 91 cast members and crew of The Conqueror, but there is plenty of circumstantial evidence.
This article from The Straight Dope website, gives more information on the links between the filming of the movies and the resulting cases of cancer.