Little breast tumors that seemed cured after surgery are more likely to come back if they are the type known as HER2 positive, US researchers reported.

They said women who have these types of tumors may need extra treatment with drugs such as Genentech Inc.'s Herceptin -- which is not standard practice now.

"Most physicians do not treat these small tumors with Herceptin," Dr. Ana M. Gonzalez-Angula of the MD Anderson Cancer Center in Houston, who led the study, told reporters in a telephone briefing.

After five years, 23 percent of patients with tiny tumors one cm (half an inch) or smaller whose cancer was HER2 positive had tumors come back after surgery, Gonzalez-Angula told the San Antonio Breast Cancer Conference.

Her team looked at more than 1,300 women between 1990 and 2003. Ten percent had HER2 tumors and they had a much higher likelihood the cancer would come back than those with the more common estrogen receptor positive tumors or those with so-called triple negative tumors.

These three types of tumor each have a different mutation that drives the cancer. Estrogen receptor positive breast tumors are the easiest to treat, with a range of drugs that affect the hormone.

The women, with an average age of 57, had 2.68 times the risk their cancer would come back after surgery if they had HER2-positive tumors than the other patients, Gonzalez-Angula told the meeting.

HER2-positive tumors had been more difficult to treat but Herceptin, known generically as trastuzumab, is a genetically engineered antibody -- an immune system molecule -- that homes in on that particular mutation.