Tumor cells found in the blood of breast cancer patients up to 20 years after surgery suggest that the body may have ways of keeping a dormant cancer in check, U.S. researchers said on Wednesday.

One third of 36 former breast cancer patients were found to have the cells, according to the team at the University of Texas Southwestern at Dallas.

The study, published in the journal Clinical Cancer Research, seems to show the body can maintain a balance between tumor cell proliferation and cell death. People who stay cancer-free may have a system for keeping a tumor under control, the researchers said.

Some of the patients had undergone mastectomies 20 years before. Such people have a low rate of cancer recurrence, but after about 20 years the risk rises sharply to 20 percent - a condition called dormancy that doctors do not understand.

“Dormancy is a mysterious phenomenon that occurs in certain types of cancer,” said Dr. Jonathan Uhr, who led the study.

His team collected blood from 36 patients who had been free of cancer for at least 7 years. Thirteen were found to have circulating tumor cells. Because these cells do not live long, Uhr said there must be tiny, unseen tumors somewhere in the body producing them.

Understanding how the body controls these tumors may offer new ways to treat and prevent cancer, he said.

“It is also important to determine what changes are responsible for relapse,” Uhr said. “If patients at risk for impending relapse can be identified, it may be possible to prevent recurrence with appropriate therapy.”