By Gene Emery

(Reuters Health) – Breast cancer can be difficult to detect in women with extremely dense breast tissue, but a new Dutch study indicates that getting an MRI scan can spot tumors that would otherwise be missed.

“This is the first really-thorough investigation into the effect of MRI on interval cancer,” said senior author Dr. Carla van Gils, referring to the tumors uncovered between screening mammograms. “There are still questions to be answered, but this is a really important step we’ve taken.”

However, 74% of the time when an MRI scan flagged something suspicious, there was no cancer. In addition, some of the cancers are unlikely to cause problems for the patient and it’s not clear whether the scans will help breast cancer patients live longer.

“The ultimate test of the value of MRI screening in women with extremely dense breast tissue will be whether its use improves survival – an answer that we will not have for a very long time,” said Dr. Dan Longo, an editor at the New England Journal of Medicine, where the new study appears, and author of an editorial accompanying the report.

But Dr. Richard Wender, chief cancer control officer at the American Cancer Society, said he disagrees with Longo’s suggestion that the study was finding too many tumors that may not have been serious enough to treat and that MRI might not improve survival.

“We’ve already proven that screening saves lives,” reducing mortality by 40%, he told Reuters Health in a telephone interview. The group given an MRI had tumors that were spotted sooner than they normally would “and had more favorable tumor characteristics. And these are cancers that were very likely to be progressive and significant.”

Women with dense breast tissue not only face a higher risk of breast cancer, their tumors are harder to spot on a conventional mammogram.

The research raises the question of whether women with very dense breast tissue, about 8% of women, would be better off getting periodic MRIs instead of mammograms. But, van Gils said in a telephone interview, “I don’t think we can conclude that from these results.”

The ongoing study, known as DENSE, involved 40,373 women ages 50 to 75 with extremely dense breast tissue and no evidence of cancer on screening mammography. Researchers invited 8,061 to get a free MRI scan of their breasts as well; 59% agreed.

The interval-cancer rate was 0.25% for the women assigned to the MRI group versus 0.50% of the women who had received a mammogram but no MRI.

The cancers that were detected also tended to be early-stage tumors and tumors that had not spread to the lymph nodes.

When the study only looked at women who actually got an MRI, the detection rate was 4 times higher than with mammography alone, Wender said.

Among the 4,783 women who did get an MRI, 9.5% were called back for a biopsy and cancer was detected in just under 1.7%, for a false-positive rate of 8.0%.

In contrast, the rate of false positives when mammography is done on women with extremely dense breasts is only about 2.5% in the Netherlands, noted van Gils, a professor of clinical epidemiology of cancer at University Medical Center Utrecht. “The number is larger with an MRI examination. It’s something we want to work on.”

The MRI false-positive rate may improve, she said, as the women in the MRI group return for two more scans at two-year intervals.

“What we’re already seeing in the first round of screening is a learning effect,” she said. “What’s likely to happen is, if a woman comes in for a second or third MRI, the radiologist will have prior MRI material to look back on, and it becomes easier to see if something has changed or not. That will help.”

Wender predicted that the DENSE study is “going to contribute to a sequence of studies that will change practice over time. We now have a study showing that we can find interval cancers with MRI at a pretty substantial rate.”

“We now have some direction on something that may prove promising,” he said. “Now we have to see if we can make it practical.”

SOURCE: The New England Journal of Medicine, online November 27, 2019.