CHICAGO - Here's another reason for getting a second medical opinion: Biopsy specialists frequently misdiagnose equivocal breast tissue, potentially leading to too-aggressive treatment for some women and under-treatment for others, a study suggests.

The results indicate that pathologists are very good at determining when invasive cancer is present in breast tissue, but less adept at making the right diagnosis with less serious conditions or when biopsied tissue is normal.

The study involved 115 U.S. pathologists and 240 breast biopsy specimens. Their diagnoses were matched against those of three experts. It was an experiment and may not reflect what happens outside a research setting, but the authors say the results highlight the challenges of accurately interpreting tissue under a microscope.

The study was published in Tuesday's Journal of the American Medical Association.

About 1.6 million breast biopsies are performed each year nationwide, typically after radiologists spot something suspicious on a mammogram. Tissue is withdrawn through a needle or from a surgically removed growth and examined under a microscope.

Previous research has shown that interpreting mammograms can also be tricky and lead to under- or overtreatment.

The new study found that pathologists correctly diagnosed "atypia" cells - which are abnormal but precancerous - only about half the time, or no better than a coin toss, said lead author Joann Elmore, a University of Washington researcher.

Treatment for this condition typically includes frequent monitoring and sometimes medication. About a third of atypia cases were misdiagnosed by the pathologists as not worrisome or normal, while 17 percent that they called atypia were deemed normal by the experts. Since as many as 160,000 U.S. women each year are diagnosed with this condition, the results suggest many may be getting inappropriate treatment, Elmore said.

Pathologists also mistakenly found something suspicious in 13 percent of normal tissue.

They had similar trouble with a condition called DCIS, in which cancer is confined to the milk ducts. Thirteen percent of these cases were misdiagnosed as less serious, while 3 percent were mistaken for invasive cancer. DCIS, diagnosed in about 60,000 U.S. women each year, has been increasing because of rising mammogram use; it can sometimes become invasive so usual treatment is surgery and radiation.

"As a woman, I would probably want to get a second opinion" with a diagnosis of abnormal pre-cancer or DCIS, Elmore said.

A JAMA editorial notes that the study lacks information on patient outcomes, so there's no proof that the experts made the correct diagnosis. Also, pathologists weren't allowed to consult with colleagues when they were uncertain about findings - while in the real world those consultations happen frequently, said editorial coauthor David Rimm, a Yale University pathologist who also interprets biopsies.

Still, he said the results are troubling and highlight that pathology is an imperfect science. Rimm, who has provided second opinions, said patients can request another opinion though their primary-care physicians.

The editorial says the results "should be a call to action for pathologists and breast cancer scientists" to improve and refine definitions of abnormalities.