A new blood test can show if cancer will return a year before it appears on X-rays or CT scans.
Scientists believe it will allow doctors to target the tumor earlier and raise survival chances.
The trial was on lung cancer but experts believe the breakthrough will apply to all types.
The researchers, who published their work in the journal Nature, took samples from tumors removed in surgery and analyzed the defective DNA.
Blood tests were taken every three months after the surgery to see if clusters of cells re-emerged. The tumors are thought to have a volume of just 0.3 cubic millimeters when the test catches them and thus can’t be detected by either X-rays or CT scans.
Experts say lung cancer relapse occurs in up to half of all patients. The new tests have alerted 13 patients of a recurrence and given others an all-clear.
A second project found tumors with “chaotic” DNA were more likely to come back, and hide from the immune system.
Experts hope understanding this will aid drug development.
Cancer Research UK said the studies were an important step in the right direction.