According to the American College of Radiology, dense breast tissue can make finding a tumor difficult.
Bend Memorial Clinic radiologist Dr. Jim Ockner says in a mammogram, fat looks dark grey or black.
Fibris and glandular tissue is white.
"If it's more dense there's more white on the mammogram, and it can be harder to see pathology, a cancer or something like that," Ockner said Friday.
State Sen. Tim Knopp, R-Bend, said,, "There were women that came to us, and said that they ended up having breast cancer, and they didn't know that they had dense breast tissue."
One of those women is a Democrat lawmaker who co-sponsored the bill. Another, a woman who had received a terminal diagnosis.
To try and make sure this never happens again, Oregon lawmakers voted unanimously last year to require doctors to tell patients if they have extremely dense breast tissue, and that they might need additional screening.
Knopp was one of the co-sponsors.
"For all of us, it was really a wake-up call that we could do more," Knopp said.
The American College of Radiology says Oregon is one of 13 states "doing more," with laws now requiring breast density notification.
"When I see a dense breast, I'm going to be a lot more careful," Ockner said. "And if there's something that I see, I'm going to recommend a follow-up."
Which means either talking with a doctor, getting an MRI or an ultrasound.
"I definitely think it will save lives," Knopp said. "There's no question that over time, this will save dozens, if not hundreds of women's lives."
Knopp says there is no criminal penalty if a doctor does not notify a patient. However. it could set up the potential for a lawsuit.