A new genetic test appears to be effective for identifying women diagnosed with the earliest form of breast cancer who are least likely to suffer a recurrence, researchers reported Tuesday.

The findings indicate the test could help women wrestling with one of the most difficult questions they face after being diagnosed: whether to undergo radiation and other treatment after a lumpectomy or whether they can skip the follow-up treatment.

About 45,000 U.S. women each year are diagnosed with ductal carcinoma in situ (DCIS), which is a very early form of breast cancer or pre-cancer often picked up by mammograms.

Some women who get this diagnosis undergo a mastectomy, but many instead opt for a lumpectomy, which is a less aggressive form of surgery that spares their breast. For those at low risk of a recurrence, that’s all the treatment they need. But at the moment there’s no good way to identify those who are at low risk and those who should undergo follow-up treatment with radiation and drugs such as tamoxifen because they are at high risk of experiencing a recurrence. As a result, most women get radiation and other treatment, which is unpleasant, can have side effects and prolongs their treatment.

The new test was developed by Genomic Health Inc. of Redwood City, Calif., which is already selling a test known as Oncotype DX to evaluate the risk of recurrence for women with invasive breast cancer. The new test, called Oncotype DX DCIS Score, tests the activity of 12 of the 21 genes used in the original Oncotype DX test to determine the risk of recurrence in DCIS patients.

In the new study, Lawrence J. Solin of the Einstein Medical Center in Philadelphia and colleagues evaluated the test on 327 women who were diagnosed with DCIS between 1997 and 2000 and followed for at least 10 years as part of an Eastern Cooperative Oncology Group study. In findings being reported at the San Antonio Breast Cancer Symposium this week, Solin reported that testing the patients’ tumors reliably identified the risk that women would develop a recurrence.

About 75 percent of the patients had a low score, indicating they had only about a 12 percent chance of experiencing a new invasive cancer or a recurrence of DCIS in the same breast, and could consider forgoing follow-up therapy, the researchers found. Those with a high score had a 27 percent chance of a local recurrence, of which about half were likely to develop a new invasive breast cancer, the researchers reported.

“This is very exciting,” Solin said during a telephone interview. “Think about how powerful it would be to be able to tell women, ‘You’re risk of a recurrence is X percent.’ That’s a huge advance.”

Genomic Health plans to begin making the test available Dec. 28, according to Steven Shak, the company’s chief medical officer. It will cost $4,175, and the company hopes most insurance companies will cover it. It takes a week or so to get the results, he said.

Other experts said the test could provide a very useful tool for many women.

“It will give clinicians and women more information to inform their approaches to treatment,” said Susan Love, a breast cancer expert, in an e-mail.

Others, however, cautioned that more research should be done to validate the test.

“I don’t think the test is ready for prime time,” said Minetta Liu of Georgetown University in an interview. “It would be good to look at another data set.”