Robotic surgery was never approved for mastectomy or any other cancer-related treatment, but that has hardly deterred doctors in the operating suite. The equipment is widely used to operate on patients with various malignancies, from breast cancer to prostate cancer.

Yet there have long been questions about how well doctors are trained on the machines, and whether the devices are better for patients than traditional methods.

Now the Food and Drug Administration has warned that there is no evidence cancer patients receiving robotic procedures live longer than those who have traditional procedures. And some research shows that patients with cervical cancer fare worse.

“We want doctors and patients to be aware of the lack of evidence of safety and effectiveness for these uses so they can make better-informed decisions about their cancer treatment and care,” said Dr. Terri Cornelison, assistant director for the health of women at the agency’s Center for Devices and Radiological Health.

Robotic systems have been on the market for more than 15 years and have been used for cancer surgery for much of that time. The machine’s tower, which is positioned over the patient, looks a bit like a multiarmed “Star Wars” droid.

The robot’s arms are controlled by a computer that replicates the movements of the operating surgeon, who manipulates the robot’s controls while looking at a monitor that provides a magnified, high-definition image of the operating site.

The FDA noted the findings of two studies published last year in the New England Journal of Medicine.

One of them was a clinical trial that was stopped early after investigators found that women with cervical cancer who had minimally invasive hysterectomies, including robotically assisted procedures, experienced four times as many cancer recurrences and six times as many deaths compared with patients who had the more traditional procedure.

The trial’s findings were especially striking because the surgery, a radical hysterectomy, usually cures patients with cervical cancer, said Dr. Pedro T. Ramirez, lead author of the paper and director of minimally invasive surgical research at the M.D. Anderson Cancer Center in Houston.

A second study funded by the National Institutes of Health used a database to compare the outcomes of 2,461 women with cervical cancer who had different types of surgery. Four years after the operation, 9.1% of those who had minimally invasive surgery had died, compared with 5.3% of those who had open surgery.

Yet many physicians continue to recommend robotic surgery to patients, despite the evidence of harm.

The FDA’s warning was not limited to cervical cancer: Robotic surgical devices are not authorized for prevention or treatment of any cancer, the agency said.