“I was trying to focus in on who’s really at high risk of having their breast cancer not seen on mammography,” said Dr. Karla Kerlikowske, the first author of the study and a professor of medicine and epidemiology and biostatistics at the University of California, San Francisco. “It can’t be 45 percent of people.”

Dense breasts have a relatively high proportion of glandular or connective tissue, which shows up as white on mammograms. Tumors also appear white, so dense tissue can hide them. Non-dense breasts have more fat, which looks dark on mammograms, so tumors stand out more easily.

Density can be detected only by mammograms and is reported in one of four categories, from “almost entirely fatty” to “extremely dense.”

The study findings are based on the medical records of 365,426 women ages 40 to 74 who had screening mammograms from 2002 through 2011. The researchers wanted to know if they could detect whether, among all the women with dense breasts, any distinct subgroups had a higher risk than others of developing an “interval” cancer, meaning one that is found less than a year after a normal mammogram.

Such tumors, usually found when a patient or her doctors feel a lump, may have been present but not detected by the mammogram.

The report uses breast density assessments along with an online calculator that estimates a woman’s risk of developing breast cancer in the next five years. The calculator asks five questions: a woman’s age, her race, breast density as reported from mammograms, whether she has ever needed a breast biopsy and whether her mother, sisters or daughters have ever had breast cancer.

The calculator categorizes each woman’s risk as low, average, intermediate, high or very high. A five-year risk up to 1.66 percent is considered low to average; more than 4 percent is very high.