Having an MRI scan before breast cancer surgery may not cut a woman’s risk of needing a second operation to go after additional tumor tissue - and may even raise her chance of getting an unnecessary mastectomy, a new study suggests.

Researchers found that among more than 300 women who underwent breast cancer surgery, those who had an MRI beforehand were no less likely to need a repeat operation to remove more tumor tissue: 19 percent did, versus 18 percent of women who did not have an MRI.

The findings, reported in the Archives of Surgery, back up past research showing that pre-op MRIs do not cut reoperation rates.

In fact, that MRI may make the cancer look worse than it actually is - raising the possibility that some women will opt for a mastectomy when they might otherwise have chosen “breast-conserving” surgery.

It has become “almost routine” for doctors to do a pricey MRI scan before a woman has surgery for breast cancer, according to Dr. Sharon S. Lum, a cancer surgeon at Loma Linda University School of Medicine in California.

That’s because the scans are more sensitive than mammograms and may detect “hidden” cancerous tissue. “MRI is very good at picking up additional disease,” said Lum, who wrote an editorial published with the study.

So, logically, it might seem that a pre-op MRI would help ensure that surgeons get all of the cancer - and then hopefully cut the need for repeat surgeries, or even the risk of a cancer recurrence.

But based on studies so far, that isn’t what has been happening.

“Now that we’ve seen it in practice, it doesn’t seem to be doing the things we’d hoped,” Lum said.

“There are many breast surgeons who routinely use MRI,” said Dr. Jan H. Wong, a cancer surgeon at East Carolina University in Greenville, North Carolina, who worked on the study.

And there are also many women who want a pre-op MRI, both Wong and Lum said.

But it’s important for women to know that the test comes with risks, they said.

For their study, Wong and his colleagues reviewed the cases of 313 women who underwent breast cancer surgery with the same surgeon.

Of those patients, 120 had a pre-op MRI because they had dense breast tissue - a group of women who, in theory, would be more likely benefit from a sensitive MRI scan because dense breast tissue can obscure tumors in mammography images.

Despite that, though, women who underwent MRI were no less likely to need a repeat operation.

“Even in this selective group,” Wong said, “we didn’t see a benefit.”

What’s more, of 47 women who had a mastectomy after their MRI, one-quarter might have been candidates for breast-conserving surgery if they’d wanted it.

That was based on pathology reports done after the mastectomy: For 25 percent of women, the cancer was less extensive than the MRI had indicated.

“The problem is, when you have a sensitive test, it picks up abnormalities that aren’t related to the cancer,” Wong explained. So the doctor has to delve further to make sure those MRI abnormalities are “OK” - or a woman may opt to skip further tests and go for a mastectomy.

It’s a controversial area, Wong noted. Not only does pre-op MRI come with potential downsides, but it’s also very expensive; the costs vary, but can reach a few thousand dollars.

It’s still possible that some women might benefit from an MRI ahead of surgery, according to Lum and Wong.

An example would be women who carry certain gene mutations that put them at high risk of breast cancer. If they’re considering having breast-conserving surgery instead of a mastectomy, an MRI image might give some additional information to help with that decision. (MRI is already used as part of breast cancer screening for those women.)

But, Lum said, “I don’t think we have the criteria yet for who might benefit.”

And for most women, there’s a lack of evidence that pre-op MRI will ultimately affect their treatment outcome.

Both Lum and Wong suggested that women carefully discuss the risks and benefits of a pre-op MRI with their doctor.

The “assumption” that the scan is beneficial, without a substantial downside, has not been borne out by the evidence, Wong said.

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SOURCE: Archives of Surgery, September 2012.