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They’re far too small to be seen by even the most sophisticated medical imaging device. But bits of tumor DNA adrift in a patient’s blood are an important bellwether for whether cancer will return — and, as a new study shows, can predict which treatments are likely to work.

Scientists reported in the journal Nature Medicine on Monday that colorectal cancer patients who had tumor DNA in their blood after surgery were far more likely to have their cancer return. But the key finding was that it was the patients with this circulating tumor DNA who needed and benefited from chemotherapy, regardless of their cancer stage or other risk factors, while those without this DNA didn’t benefit from chemo.


The report’s authors think the findings could one day reshape the treatment of a disease that ranks third in the U.S. in both new cancer cases and deaths. They argue the benefits of their blood-based approach are twofold. Patients without circulating tumor DNA could be spared from chemo, and from the fear their cancer will come back. And those with bits of tumor DNA in their blood could receive treatments likely to benefit them — with continued monitoring to ensure those therapies are working.

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