WEDNESDAY, Jan. 19 (HealthDay News) -- For decades, doctors have debated about how to best determine whether a patient's breast cancer has spread, especially in the earliest cases of metastases.

Now, researchers have compared two approaches -- a sentinel lymph node biopsy alone, or the sentinel biopsy combined with axillary dissection, which is a more invasive procedure that can spot hidden, smaller metastases. The finding: As long as the larger metastases (2 millimeters in diameter and up) are found, the outcomes of the two procedures are similar.

"What we showed was the significance of these small micrometastases is very small," explained study author Dr. Donald L. Weaver, a professor of pathology at the University of Vermont College of Medicine and Vermont Cancer Center. The report is published in the Jan. 19 online edition of the New England Journal of Medicine.

Weaver and his team randomly assigned 5,611 women with breast cancer but clinically negative axillary nodes to one of two groups -- about half underwent the sentinel node biopsy alone and the other half underwent the biopsy plus axillary dissection.

A sentinel lymph node is the first lymph node to which cancer is likely to spread. The biopsy is based on the idea that cancer cells metastasize in an orderly fashion.

At the centers participating in the study, the sentinel node exam was designed to find all metastases more than 2 millimeters in dimension, known as macrometastases.

Follow-up data was available for 1,924 in the combination group and 1,960 in the biopsy-only group. In the biopsy-only group, 300 were positive for metastases, while 316 were positive in the combination group.

Of those who had metastases, 172 were micrometastases, 14 had macrometastases and 430 had even tinier spreads, known as isolated tumor-cell clusters, the study authors found.

The researchers looked at differences in the patients in whom the hidden metastases were found and in those in which they weren't found. They compared overall survival, disease-free survival and distant-disease-free survival.

"If you had them, the [overall] survival was 94.6 percent; if not, 95.8. There's only a 1.2 percent difference between the two," Weaver said.

While the difference was significant from a statistical point of view, it was slight from a clinical point of view, he noted.

For women, Weaver said, the take-home message is not to be concerned about metastases being missed, provided the sentinel lymph node biopsy was done.

"Enough is enough as long as you find the macrometastases, the ones over 2 millimeters," he said.

As for the smaller spreads? "They are probably being treated by whatever cancer treatment is recommended [for the primary tumor]," he said, such as chemotherapy, endocrine therapy or radiation.

The new information "kind of solidifies an idea that we know," said Dr. Laura Kruper, an assistant professor of oncology and a breast cancer surgeon at the City of Hope Comprehensive Cancer Center in Duarte, Calif.

What the findings suggest, she said, is that the standard procedure followed by many -- do the sentinel lymph node biopsy alone if it's negative -- seems to be effective.

She added, however, that "we really do need to continue to follow these patients long-term" to see if the differences remain small.

More information

To learn more about sentinel lymph node biopsy, visit the U.S. National Cancer Institute.