The study included more than 100,000 women and more than 300,000 screening exams.
"We found that screening examinations of women having dense breasts showed higher rates of recall and biopsy, and higher odds of screen-detected and interval breast cancers than women with non-dense breasts," said the study's senior author, Solveig Hofvind. She is a researcher and head of BreastScreen Norway for the Cancer Registry of Norway.
Dense breasts pose a challenge when it comes to cancer screening, because dense tissue shows up white on a mammogram. That's also how breast tumors look on a mammogram. Dense breast tissue can actually hide or mask cancers, according to Hofvind.
The findings were published June 26 in Radiology.
Dr. Liane Philpotts wrote an accompanying editorial. She is chief of breast imaging at the Yale School of Medicine.
"Dense breasts are not something that a patient feels. You can only tell if someone has dense breast tissue on a mammogram," Philpotts said.
Radiologists identify breast density using a standardized scoring technique from the American College of Radiology (ACR). The scoring system runs from A to D. A woman with an A or B doesn't have dense breasts, but someone with a C or D does, she explained.
About half of American women who are screened for breast cancer have dense breast tissue. As women age, their breasts often become less dense, Philpotts said.
Instead of using the ACR technique, which relies on a radiologist's subjective judgment, the new study used automated software -- known as automated volumetric analysis -- to classify breast density.
The Norwegian women in the study were between 50 and 69 years old. The automated software found dense breasts in 28 percent of their screening tests.
The rates of cancer were 6.7 per 1,000 exams for women with dense breasts and 5.5 for women with non-dense breasts, according to the findings.
"This study really shows that women with dense breasts did have more cancers. It wasn't a huge amount. It was a small increase, but it was an increase," Philpotts said.
In addition, women with dense breasts had more interval cancers. These are cancers found between screenings -- for example, when a woman feels a lump in her breast.
The study found that women with dense breasts were called back for more testing due to suspicious findings and were more likely to have a biopsy to check tissue for cancer than women without dense breasts.
Women with dense breasts also tended to have larger tumors when cancer was detected -- average of 17 millimeters (mm) vs. 15 mm for women without dense breasts.
The study also confirmed that it's harder to accurately identify breast cancers in dense breast tissue. Cancers were accurately detected in women with dense breasts 71 percent of the time compared to 82 percent for women without dense breasts.
"Automated volumetric breast density measurements may be considered a future standard for breast cancer screening, ensuring an objective density classification," Hofvind said.
Philpotts pointed out that the findings don't necessarily translate to a U.S. population, because the women screened in the study were older, and they were screened every other year instead of annually.
She said more research is needed to gauge the risks and benefits of the automated software. Hofvind agreed.
Women with dense breasts generally don't need to be screened more often, according to Philpotts. But they will need some sort of supplemental imaging such as ultrasound or MRI that's better at seeing the difference between dense tissue and cancerous tissue.
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