The standard "all-clear" letter sent after mammograms to tell women they are cancer-free is going to contain new and potentially troubling information for thousands of Minnesota women — the disclosure that they have dense tissue in their breasts that could cloud their cancer screenings.

Minnesota mandated as of Aug. 1 that doctors notify women if their mammograms discover dense breast tissue, which can mask the presence of a tumor on an X-ray.

"There is no shortage of women like me who have been harmed by their density and never knew it," said Nancy Cappello, a breast cancer survivor from Connecticut. She became an advocate for state-mandated disclosure of tissue density after mammograms failed to find her tumor until it was enlarged and her cancer had spread beyond her breast.

Breast tissue density is well-known among cancer specialists and radiologists; studies suggest 47 percent of women have tissue that is dense enough to hide a tumor on an X-ray image. Research also has indicated that dense breast tissue itself increases cancer risks, especially for the 8 percent of women with extremely dense tissue.

But some doctors treat tissue density as back-of-the-house information that patients don't need to know, in part because it is unclear exactly what women should do with the information and which alternative cancer screenings work best.

Cappello said doctors were so resistant to notifying patients on their own that she formed the "Are You Dense?" advocacy organization and sought mandates that have been enacted in 19 states.

"I was shocked," she said, "not that I had breast cancer, but that it was [detected] so darn late."

While mandate proposals have generated intense debates in other states, the Minnesota legislation quietly received support in the House this spring before it was tucked into a health care omnibus bill and sent to the governor's desk. Organizations such as the Minnesota Medical Association had concerns — especially the impact of notifying so many women when the solutions for dealing with dense breast tissue are unclear — but didn't oppose the bill.

Christine Norton of the Minnesota Breast Cancer Coalition suspects the lack of opposition was due to a "pink tsunami" of high-profile breast cancer advocacy campaigns, which make it politically difficult to oppose an increase in screening even if it might lead to more incidents of misdiagnosis and overtreatment.

Norton opposed the mandate because there is no scientific proof yet that supplemental screening beyond mammography reduces the rate of breast cancer deaths. Diet and alcohol consumption are known risk factors for breast cancer, but they might not receive as much emphasis if doctors are busy explaining breast tissue density to patients, she added.

"Shouldn't women get notification that they're overweight? It also is a risk factor. And that's something a woman can change," Norton said.

Only a fraction of women with dense tissue have breast cancer. Some women, when told, ignore it or simply increase the frequency of manual self-exams, Norton said. Others stop taking hormone replacements because of evidence that it increases breast density.

Screening alternatives include ultrasound imaging or a special 3-D ultrasound called digital breast tomosynthesis, which has been shown to increase breast cancer diagnoses while reducing false positive results. Women with family histories of breast cancer are supposed to seek more accurate MRI scans, but they are not recommended for other women.

No national guidelines exist on which approaches to use. The U.S. Preventive Services Task Force is examining breast tissue density as it updates its guidance on breast cancer screening, but new guidance isn't expected for months.

Current recommendations

The task force currently recommends mammograms every other year for women ages 50 to 74, and screenings at earlier ages depending on family cancer histories.

Confusion over screening alternatives shouldn't excuse doctors from their responsibility to notify patients of tissue density, Cappello said. She was in Minneapolis Wednesday to speak with doctors at the invitation of Delta Medical Systems, a medical supplier that sells ultrasound systems.

Sarah Lewerenz, a 57-year-old attorney from Duluth, said her tissue density fooled mammograms to the point that a tumor in her breast was the size of a lemon before it was discovered in 2011.

"The opportunity to talk to my physician about what it meant to have dense breasts could have been important to me," said Lewerenz, who will forever wonder if earlier detection could have meant treating the cancer with a simple removal procedure rather than multiple surgeries, chemotherapy and radiation.

A key question regarding the mandate is whether insurers will cover alternative screenings for women with dense tissue.

Blue Cross and Blue Shield of Minnesota, the state's largest insurer, doesn't have a policy but "works closely with providers and supports their judgment for when additional evidence-based imaging studies may be necessary," said Dr. Paul Karazija, Blue Cross' vice president and executive medical director for commercial and government programs.

Letters with mammogram results started going out from Mayo Clinic this month with information on tissue density. The response has been so mild that the clinic's director, Dr. Karthik Ghosh, wonders if women are reading beyond the first sentence indicating a negative test result.

"You know what happens with those letters," she said.

Ghosh said mammograms are 88 percent accurate for women with little tissue density but still 62 percent accurate for women with extreme density, so women should continue to undergo testing. On the other hand, she noted, women shouldn't dismiss lumps in their breasts just because their mammograms were negative.