TUESDAY, May 3 (HealthDay News) -- Breast tumors that are detected in between regular screening mammograms tend to be more aggressive and fast-moving than those found during scheduled screenings, indicating that better screening methods are needed, researchers say.

"We may have to look at other imaging techniques or newer technologies to find these types of cancers," said Anna M. Chiarelli, co-author of a study published online May 3 in the Journal of the National Cancer Institute.

Other studies have come up with similar results but they didn't make a distinction between what these authors call "true" interval cancers, meaning those that weren't detectable by a radiologist on a regular screening mammogram but showed up in retrospect, as opposed to "missed" interval cancers, which did show on the previous mammogram but were missed.

"Interval cancers are a heterogeneous group," said Chiarelli, a senior scientist in prevention and cancer control at Cancer Care Ontario in Toronto, Canada. "We wanted to see the different pathologies [and] prognostic factors."

In this study, an interval cancer was one that was diagnosed within two years after the last negative mammogram.

The authors identified "true" interval cancers and "missed" interval cancers in a group of almost 431,500 Ontario women aged 50 or older who were screened from 1994 through 2002.

They then compared those tumors with a control group of mammography-detected cancers.

Both the true and missed interval cancers were more aggressive than the cancers picked up by screening in that they were a higher stage and grade.

But the true interval cancers had even more worrying characteristics -- they were three times more likely to have a high "mitotic index," which is a measure of how fast the cells are dividing. They were also more than twice as likely to be both estrogen-receptor and progesterone-receptor negative, which means that they cannot be fought with hormonal therapies and so are usually harder to treat.

True interval cancers were also more likely to belong to the minority of breast cancers that do not originate in the milk ducts.

"The true interval cancers were even worse than the missed intervals," Chiarelli said.

It's possible the missed cancers had worse characteristics than the true intervals simply because they were caught later, she added.

One expert said the phenomenon might not be so common in the United States, however.

"In Canada they only do mammograms every two years, so the number of interval cancers in the U.S. is going to be smaller because we're doing mammograms every year," said Dr. Paul Tartter, a senior breast surgeon with St. Luke's-Roosevelt Hospital's Comprehensive Breast Center in New York City. "Also, all of these mammograms were done before digital mammography, so hopefully digital mammography will reduce the number of missed cancers," he said.

Ultrasound detection is one other option that women might have available to them to spot cancers, said Dr. Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in New York City. But, she added, "it's hard to prove a large benefit to ultrasound."

Still, when interpreted by the right person, "it's definitely useful in women with dense breast tissue," Bernik added.

But although women with denser breast tissue might benefit from this additional screening, it's often not covered by insurance, she noted.

"The good news is that interval cancers are a small percentage of cancers," Chiarelli said. "Most cancers are found from [mammographic] screening but it's important to look at these and to have to find a technique to detect them."

In contrast to a recent guideline change, Bernik advised that women should start getting mammograms when they're 40 years old.

More information

The U.S. National Cancer Institute has more on breast cancer screening.