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SACRAMENTO  (AP) — California women who have dense breast tissue will get more information following their routine examinations under a state law that took effect Monday.

Women with that type of tissue must be notified of that finding after a mammogram. Health professionals say as many as four in 10 women over the age of 40 have dense breast tissue, which can make it more difficult to detect cancer.

Patients with such tissue also will be told that their doctor can recommend additional screening options.

The new requirements are due to a bill authored by former state Sen. Joe Simitian, a Democrat from Palo Alto. One of his constituents, Amy Colton of Santa Cruz, suggested the measure after annual mammograms did not reveal her breast cancer until it had reached an advanced stage.

"It's one of those times where what you don't know can hurt you," said Simitian, who is now a Santa Clara County supervisor.

He pushed an initial version of the bill in 2011, but it was vetoed by Gov. Jerry Brown. The governor cited concerns about raising "unnecessary anxiety" among healthy patients.

The California Medical Association and other physician groups were concerned about the phrasing of the required notice to patients in Simitian's original bill. The association dropped its opposition after he reworked the legislation.

Bob Achermann, executive director of the California Radiological Society, said the initial version suggested more strongly that additional screenings need to be done if a woman has dense tissue. A number of factors must be considered and discussed with the patient's doctor, he said.

Brown signed the second attempt into law last September. Similar notification laws have been enacted in Connecticut, Texas, New York and Virginia.