Blood samples can effectively monitor cancer and help doctors better prescribe treatment, a study said Saturday.

Currently tumor biopsies are generally used to assess changes in a cancer's DNA. New advances may allow researchers to study cancer via the bloodstream where tumor cells shed small, detectable pieces of their DNA, researchers said.

In a study presented at the annual meeting of the American Society of Clinical Oncology, researchers used blood samples from more than 15,000 patients and 50 types of tumors with a test technology invented by California-based Guardant Health, which looks for mutations in 70 cancer-related genes.

By assessing the DNA, doctors can "monitor changes in the cancer as it evolves over time, which can be critical when patients and physicians are discussing treatment options for continued tumor control," the study says.

"These findings suggest that analysis of shed tumor DNA in patient blood, also known as a liquid biopsy, can be a highly informative, minimally-invasive alternative when a tissue biopsy is insufficient for genotyping or cannot be obtained safely," said study presenter Philip Mack, professor and director of molecular pharmacology at the University of California Davis Comprehensive Cancer Center.

Blood tests can also provide a more comprehensive sample of a tumor's DNA since biopsy tissues come from only one part of a tumor and therefore may not provide a sample of the changing DNA.

"Having a good, reliable option beyond a tumor biopsy could have a major impact on our ability to select the right therapy for the right patient," said Sumanta Kumar Pal, a medical oncologist at the City of Hope cancer center in California, who was not involved in the research.