Doctors tell patients that regular mammograms are the best way to find breast cancer early, but Nancy Cappello's story suggests women may want to ask more questions.
Cappello went for a mammogram every year for a decade.
"I have no risk factors that I knew of or first degree relatives with breast cancer, and I did my breast exams," Cappello said.
So she was shocked when, weeks after being told her mammogram was normal, her doctor felt a lump. It was invasive breast cancer.
"I don't get why the mammogram didn't find my cancer," said Cappello, who was diagnosed in 2004 at age 51.
Her doctors told her it was because she had dense breast tissue, which can make detecting cancer more challenging.
More than 40 percent of women have dense breasts but many are never told that their type of breast tissue makes cancer harder to spot on a mammogram.
A woman's breasts are made up of fatty, fibrous, and glandular tissue, according to the American College of Radiology and the Society of Breast Imaging. Shape and size don't indicate density -- breasts are considered dense if a woman has a lot of fibrous or glandular tissue, but not much fat. Doctors classify breast density using a four-category scale.
Dr. Regina Hooley, associate professor of diagnostic radiology at the Yale School of Medicine, told CBS News that with a mammogram, trying to find a tumor in a dense breast is like trying to find a snowball in a snowstorm.
"The dense breast tissue appears white on the mammogram. Cancers also appear as white spots on the mammogram," she said.
And that's why doctors may recommend a follow-up screening using ultrasound, which can help experts spot cancers more easily.
For high risk patients, doctors might also suggest magnetic resonance imaging (MRI). It's the most sensitive test, but also the most expensive, according to the American College of Radiology and the Society of Breast Imaging. Some health insurance may not pay for additional testing either.
Supplemental screenings carry the risk of false positives, but Hooley says those errors are declining.
"Our false positive rate has decreased by about a half," she said.
Cappello started a nonprofit organization, Are You Dense?, to educate women and push for change when it comes to screening for breast cancer.
"We've got to do something better. We have too many women who are dying," said Cappello, who is now monitored closely for any signs of cancer.
In 2009, her work led her home state of Connecticut to pass a law requiring radiologists to inform women if they have dense breasts and mention ultrasound or MRI as possible follow-ups. Two dozen states now have density notification mandates and a federal bill is making its way through Congress.
Dense breast tissue is common in younger women, and the American College of Obstetricians and Gynecologists says women with dense breasts have a "modestly increased" risk of breast cancer.
However, the group does not recommend routine use of additional tests beyond screening mammography in women with dense breasts who are asymptomatic and have no additional risk factors. The organization does recommend that health care providers follow state laws that may require them to inform women of their breast density based on a mammogram report.