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NEW YORK - Some women who have lumpectomies for breast cancer may then undergo second operations they do not need, because guidelines for deciding who requires repeat surgery are unclear, a new study finds.

It also hints that some women who might benefit from further surgery may be missing out on it.

The additional operations are done when pathology reports on tumor specimens suggest that the first operation may have left behind some cancer cells. But surgeons differ when it comes to interpreting those reports.

Such uncertainty about a cancer operation that has been in use for 30 years is “a shame,’’ said Dr. Laurence E. McCahill, the first author of the study, and a surgeon and assistant director of the Lacks Cancer Center in Grand Rapids, Mich.

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Dr. Monica Morrow, the chief of breast surgery at Memorial Sloan-Kettering Cancer Center in New York, said, “It is getting to be the time for leaders in radiation oncology and surgery to get together and make a consensus statement that could help to guide their membership.’’

McCahill’s study, published online yesterday by the Journal of the American Medical Association, is based on the medical records of 2,206 women who had lumpectomies at one of four hospitals in different parts of the country. Overall, 22.9 percent had more than one operation.

Nearly half the repeat operations were done in women whose pathology reports did not indicate that any stray cancer cells had been left behind, meaning that the operations probably did not help the patients. More disturbingly, 14 percent of patients who had evidence of cancer left behind did not have another operation, for unknown reasons.

Rates of repeat surgery varied by surgeon, from zero to 70 percent, and by hospital, from 1.7 percent to 20.9 percent. Differences in the patients did not account for most of the variation, although very thin women, who tend to have small breasts, were more likely to need second operations. Surgeons say that is because they try hard to remove as little tissue as possible from small breasts, for cosmetic reasons.

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“The number of operations women undergo definitely depends on where you’re treated and even to a greater level on which surgeon you see,’’ McCahill said. “We put the math behind it and said, this is a lot of second operations that maybe don’t make a difference.’’

A major reason for the variation is that there is no consensus among surgeons about how big a rim or “margin’’ of healthy tissue should be taken out when a cancer is removed. Surgeons try to cut cleanly around a tumor and remove enough of a margin to ensure that they excised all the cancer.

Negative or clean margins generally mean the whole tumor was removed. Positive margins mean some cancer was probably left behind and another operation is needed to prevent recurrence.

But how big should the margins be? When the tumor and the healthy tissue are removed, the surface of the whole specimen is inked so that when it is sliced and seen through a microscope, the pathologist can use the ink marks to find the margins between the cancer and the healthy tissue and measure how close the cancer cells are.

Some surgeons consider the operation complete if no cancer cells are touching ink. Others want a wider margin of healthy tissue between the cancer cells and the ink: 1, 2, or even 5 millimeters (a millimeter equals 0.04 inch), and if there is less, they operate again. Surgeons say some radiation and medical oncologists insist on a certain margin size before starting the rest of the patient’s treatment. The decision to reoperate also depends in part on what type of tumor the patient has.

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But there is no evidence that a margin any bigger than “not touching ink’’ affects cancer recurrence or survival, said Morrow, who wrote an editorial accompanying the study. At her hospital, she said, “not touching ink’’ is generally considered good enough.

Many doctors think that bigger margins must be safer, Morrow said, but studies indicate they are not.

“We really could decrease a significant amount of surgery that women are getting if we could come to a consensus that in this era, bigger is not necessarily better,’’ Morrow said.

Nearly all women now receive additional treatment after lumpectomy - radiation and systemic treatment with chemotherapy or hormones, or both - and those treatments have greatly lowered the recurrence rate and made the size of margins less of a concern than in the past, Morrow said.

Doctors cautioned that it would be a mistake to look for surgeons with a low rate of repeat operations. Dr. Susan K. Boolbol, the chief of breast surgery at Beth Israel Hospital in New York City, cautioned that a low rate could mean the surgeon usually did the operation right the first time - but could also mean the surgeon did a lot of mastectomies, or failed to operate again after lumpectomy even when more surgery was needed.

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