From the time she felt a lump to the time she lost her breast to cancer seven months later, Nancy Gustafson encountered enough uncertainty to last a lifetime.

First came the shock that her tumor, which appeared benign, was not. Then it took four surgeries to clear the killer cells. She was hoping a lumpectomy and radiation would cure her. But last December, surgeons uneasy about her prognosis without a mastectomy removed her breast.

“After that, I said, ‘I want my life back,”‘ said Gustafson, a 43-year-old pharmacist from Stafford Springs, Conn.

But by the time her doctor assured her that all the tests available indicated she was cancer-free, Gustafson was not quite ready to relax.

Recently, she finally got the peace of mind she’s been seeking. An imaging test called positron emission tomography, or PET scanning, showed no signs of lingering or recurrent cancer anywhere in her body.

In recent years, PET scanning has been recognized as among the most accurate ways to detect and evaluate cancer, especially in patients who already have been treated for the disease. It also has proven effective for finding lung tumors too small to be seen by other means.

But only recently has PET become the technology of choice for tracking down recurrences of breast cancer. The technology is considered so useful that recently Medicare, the federal health insurance program for older people, agreed to cover its $2,000-a-scan cost.

“I like to use PET scans in a very optimistic way,” said Dr. Jere Licciardello, an oncologist at Grove Hill Medical Center and New Britain General Hospital in Connecticut, “to see if a patient is clean.”

NANCY GUSTAFSON IS readied by Richard Wetmore, a radiology technician, for a positron emission tomography scan at New Britain General Hospital in New Britain, Conn. The scan is to see if her recurrent breast cancer is spreading to other parts of her body.

Even if the patient does not get good news, PET scanning could prolong life by allowing doctors to better target their treatment. A study published last year in the Journal of Nuclear Medicine found that a PET scan changed the course of treatment for 60 percent of women with recurrent breast cancer.

Some women scheduled for surgery were given only radiation as a result of the PET scan, the study said. Others may have been able to avoid planned chemotherapy. In many cases, the scan results prompted doctors to switch from one kind of chemotherapy to another, the report said.

“These results demonstrate the importance of PET in making treatment decisions for women with recurrent breast cancer,” said Dr. Johannes Czernin of the University of California, Los Angeles, one of the study’s authors.

Licciardello said that while PET scanning can be useful in spotting cancer recurrences and for helping to determine if chemotherapy is targeting and shrinking existing tumors, it is not recommended as a screening test for patients without known or suspected cancer.