Women who get an increasingly popular course of targeted radiation that takes less time after a breast tumor is removed are almost twice as likely to need a mastectomy later as those who get a more complete course, a study found.

Partial radiation, known as brachytherapy, allows women to complete the twice-daily treatment in a week, compared with more than five weeks of traditional radiation. The number of women opting for the targeted course grew to 13 percent by 2007 from less than 1 percent in 2000, the researchers said.

The study, reported Tuesday at the San Antonio Breast Cancer Symposium, found 4 percent of those getting partial breast irradiation subsequently had a mastectomy, compared with 2.2 percent of those on standard treatment.

“Since the goal of all this therapy is to avoid cancer recurrence and preserve the breast, the fact that women treated with brachytherapy were more likely to lose their breast than women treated with whole breast radiation is concerning,” said Benjamin Smith, a study author and senior researcher at the University of Texas MD Anderson Cancer Center in Houston.

Researchers reviewed Medicare claims for 130,535 women diagnosed with early-stage breast cancer after menopause. They found that the “strongest predictor” of whether a woman received a mastectomy was the type of radiation received, outweighing known risks such as chemotherapy, age and the amount of cancer spread, according to the study.

The results were unexpected, said Smith. The faster intense treatment with brachytherapy, which involves inserting a catheter containing a radioactive source directly into the breast where the tumor was removed, seemed logical, he said.

The machines that deliver high-dose brachytherapy are made by companies including Varian Medical Systems, based in Palo Alto, Calif., and Stockholm’s Elekta. Catheters that are inserted into the breast include Bedford, Mass.-based Hologic’s Mammosite, C.R. Bard’s Contura and Savi from closely held Aliso Viejo, Calif.-based Cianna Medical. Only Mammosite catheters were available when the study was conducted, though newer versions have since been introduced, Smith said.

It’s unclear whether the results will apply to the newer catheters, he said.

A definitive trial comparing a variety of partial breast radiation methods to whole breast radiation is under way in more than 4,000 women, Smith said. Those results won’t be available for several years, he said.