Researchers are testing an intriguing new weapon for patients battling cancer: rigorous physical exercise.

Studies and clinical trials at Memorial Sloan Kettering Cancer Center in New York City aim to find out if a regimen of exercise training can inhibit or delay the spread of a malignant tumor and help prevent its recurrence. An early-stage trial currently under way involves 72 women with stage 4 breast cancer, which has spread to other parts of the body and is generally considered incurable.

Scientists say the research, part of an emerging field known as exercise oncology, could take years to prove a link between exercise and cancer. If successful, they hope exercise someday will become a standard of care in cancer treatment, along with conventional therapies such as surgery, radiation and chemotherapy.

Previous studies have found, for example, that breast-cancer patients who exercise have a lower risk of recurrence and are less likely to die from their disease than women who are inactive. But the findings, from observational studies, aren’t definitive, experts say. Exercise also has been shown to help some cancer patients tolerate the debilitating effects of chemo and radiation treatments.

The new research at Sloan Kettering includes randomized, controlled studies—considered the gold standard for scientific inquiry—seeking to prove that exercise can alter the biology of a tumor, thereby inhibiting or slowing its growth, says Lee Jones, who is leading the Sloan Kettering effort.

Dr. Jones, an exercise scientist with Sloan Kettering’s Cardiology Service, whose research has focused on oncology, says studies with animals suggest the idea of reversing tumor growth with exercise may be possible. A study he co-authored, published last year in the Journal of the National Cancer Institute, found that “exercise statistically significantly reduced tumor growth” in mice with breast cancer.

Some doctors caution that some cancer patients can’t tolerate even moderate exercise. Chemo drugs can take a heavy toll on the body, for example, and cancers that metastasize to the bones could raise the risk of fractures, says Anne McTiernan, a doctor and researcher at the Fred Hutchinson Cancer Center in Seattle.

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