The higher incidence of prostate, skin and other types of cancer was revealed after analysis of data gathered over the past two decades for the Air Force Health Study on Operation Ranch Hand was adjusted to take into account veterans' years in Southeast Asia.
The latest analysis found that veterans who sprayed herbicides as part of Operation Ranch Hand and who had the highest exposure to dioxin were more than twice as likely to develop cancer "at any anatomical site" than unexposed veterans who were in southeast Asia for two years or less.
The Ranch Hand veterans' risk of contracting prostate cancer was more than six times greater than that of the other veterans, while their risk of melanoma, a type of skin cander, was more than seven and a half times greater, the study found.
But Joel Michalek, an author of the analysis, cautioned that there was less statistical confidence in the figures for prostate cancer and melanoma because they were based on a small number of cases.
The analysis will be published in the February edition of the Journal of Occupational and Environmental Medicine.
Previous analyses of the air force data had found no increase in the incidence of those cancer categories among Operation Ranch Hand veterans when compared to other veterans who served in southeast Asia but did not spray herbicides.
But a couple of years ago, researchers discovered that veterans who had spent more than two years in Southeast Asia were themselves at a higher risk of cancer, whether they had sprayed Agent Orange or not.
"So in past analyses when we failed to adjust for years in the region, we failed to find the cancer effect in the control group," said Michalek.
That, in turn obscured the increase in risk to veterans exposed to dioxin when compared to other veterans who served in southeast Asia because both groups were at higher than normal risk.
The latest air force study found that Ranch Hand veterans as well as veterans who served in southeast Asia but did not spray herbicides had a higher incidence of prostate cancer than the US population as a whole.
The veterans exposed to dioxin also had a higher incidence of melanoma than the US population as a whole.