BOSTON (AP) — Researchers have taken an important step toward better lung cancer treatment by using blood tests to track genetic changes in tumors as they progress from their very earliest stages.

With experimental tests that detect bits of DNA that tumors shed into the blood, they were able to detect some recurrences of cancer up to a year before imaging scans could, giving a chance to try new therapy sooner.

It's the latest development for tests called liquid biopsies, which analyze cancer using blood rather than tissue samples. Some doctors use these tests now to guide care for patients wit...