BOSTON – Cancer turns up five times more often in women with extremely dense breasts than in those with the most fatty tissue, a study shows, signaling the importance of a risk factor rarely discussed with patients.

On mammograms, fat looks dark, but dense tissue is light, like tumors, so it can hide the cancers. But this study confirms that cancers are also more frequent – not just hidden – in women with dense breasts.

That means that density is a true risk factor, along with other strong predictors like age and the genes BRCA1 and 2. Yet specialists say that breast density is rarely considered with other risk factors in discussions between doctors and patients.

“It’s been ignored to an absolutely unbelievable degree,” said study leader Dr. Norman Boyd at Princess Margaret Hospital in Toronto.

The Canadian study by cancer centers in Toronto and Vancouver focuses on how and when cancers were found over eight years in existing records of 1,112 women collected between 1981 and last year. It is being reported in Thursday’s New England Journal of Medicine.

Breast density comes from the presence of more connective, duct-lining and milk-gland tissue than fat. But a woman can’t judge her own density; it is routinely evaluated from a mammogram.

Previous studies had linked breast density to a higher rate of cancer, pointing to both masking and a separate biological risk.

In this study, women with at least 75 percent dense breasts showed five times more likelihood of cancer than women with less than 10 percent density.

The researchers went further by calculating just how many more cancers were found at screening, within the next year, and in the years afterward. Cancers found within a year were considered likely to be present, but masked, during the earlier mammogram. But a true biological risk was seen in cancers discovered by mammogram or long afterward.

In this study, cancers were 18 times more likely in women with the densest breasts within the first year after mammograms – the masking effect.

However, cancers in women with the densest breasts were also more than three times more likely to turn up at the time of screening and after the first year following a mammogram. That confirms and helps quantify the true biological link between density and cancer.

“I think the masking thing is important, and it does happen, but the most important thing is that this is an incredible risk factor,” said Dr. Karla Kerlikowske, of the Veterans Affairs Medical Center in San Francisco, who wrote an accompanying editorial. “This probably counts for a large percentage of the cancer that’s occurring.”

Breast cancers are the second most lethal kind after lung cancers in women. About one in eight women will get invasive breast cancer during her lifetime, according to the American Cancer Society. Last year, roughly 41,000 U.S. women died of it. Worldwide, it kills about 370,000 women each year.

In this study, density of more than 50 percent accounted for 16 percent of all cancers and a quarter in women under age 56.

Robert Smith, a screening expert at the American Cancer Society, said this study and its predecessors will encourage a rethinking of cancer screening.