By Julie Steenhuysen

CHICAGO (Reuters) - The American Society of Clinical Oncology (ASCO) has issued guidelines on how cancer doctors should approach the use of new genetic tests that screen for multiple cancer genes at the same time, including counseling patients about genes whose contribution to cancer is still poorly understood.

The guidelines aim to educate doctors about the risks and benefits of new genetic tests, argue for regulation to assure quality and call for more equitable reimbursement of the cost of the tests from private and public insurers.

The falling price of genome sequencing has made it possible for cancer doctors to cheaply test for a wide variety of mutated genes that could guide treatment or predict a person's risk for cancer.

The multi-gene tests can help doctors identify ways to treat the genetic drivers of cancer or guide treatment for people born with genes that raise their cancer risk. But often the tests turn up unexpected or ambiguous results, leaving patients and doctors wondering what the results mean.

"It's like having an all-you-can-eat buffet, and is that a good thing?" said Dr. Noralane Lindor, an oncologist and geneticist from Mayo Clinic Center for Individualized Medicine and an ASCO Prevention Committee member.

Lindor was one of several authors of the guidelines issued on Monday by ASCO and published in the Journal of Clinical Oncology.

One key area in the study is the new practice of testing tumors for mutations driving cancer growth. The tests can help suggest drugs that target these mutations. But, depending on how they are done, the tests can also uncover inherited cancer genes that could affect other family members.

When doctors order these tests, ASCO recommends that patients are informed about this possibility and be given the chance to opt out of these findings if they choose.

The new guidelines also take on the area of multi-gene panel testing for cancer susceptibility, such as genes linked to breast cancer.

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The tests, produced by Myriad Genetics Inc, Ambry Genetics, Invitae Corp and Illumina Inc and others, cover roughly 25 to 40 genes suspected of contributing to breast cancer risk.

They have become increasingly popular since June 2013, when the U.S. Supreme Court invalidated patents held by Myriad on BRCA1 and BRCA2, two well-characterized genes that increase the risk for breast, ovarian and other cancers.

"There are some genes on these panels that we know almost nothing about," Lindor said.

In the hands of an oncologist who does not fully understand the findings, patients could be treated aggressively with surgery to remove her breasts or ovaries for having mutations that would never have caused them harm, said Lindor. "The possibility of doing harm is very real."

The ASCO guidelines also call for better regulation of the tests to assure quality without hampering access to them. The U.S. Food and Drug Administration is considering regulating lab-developed tests, which include most genetic tests.

Currently, these tests fall under guidelines established by the Clinical Laboratory Improvement Amendments (CLIA) of 1988. Under CLIA, such tests are not required to prove clinical validity or utility, meaning information that aids patient care.

ASCO also recommends that cancer doctors and other health professionals ordering cancer risk tests seek out additional training in genetics.

In addition, the group wants insurers to establish coverage policies that support genetic testing services to assess inherited cancer risks. Currently, such coverage is uneven, Lindor said, with Medicare, Medicaid and some private insurers putting up barriers to patients receiving testing and counseling services.

(Reporting by Julie Steenhuysen; Editing by Bill Rigby)