(HealthDay News) — If genetic tests are only done on cancer tissue, as many as half of patients may not receive the most appropriate treatment for their cancer, according to research published in Science Translational Medicine.

Victor Velculescu, M.D., Ph.D., a professor of oncology and pathology at the Johns Hopkins University School of Medicine in Baltimore, and colleagues compared the genetic information of tumor and normal tissue from 815 patients. These patients had a variety of cancers, including breast, brain, renal, gastric, lung, pancreatic, and blood cancers and melanoma. 

Three-quarters of patients had genetic mutations for which approved therapies or clinical trials had been designed, suggesting that DNA analysis could steer these people to targeted therapy, Velculescu told HealthDay. However, a comparison of cancerous and normal tissue found that about 2/3 of all the mutations uncovered by DNA scans “were a part of the patient’s normal inherited genetic variation,” he said.

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The researchers then looked specifically at mutations in genes for which a drug or cancer therapy has already been identified. They found that nearly a third of these changes also were false positives — harmless genetic variations that had nothing to do with a person’s cancer. Each patient had multiple harmless genetic mutations, according to Velculescu. Overall, he said, the false positives affected almost 1 in every 2 patients.

Based on these findings, the researchers argued that doctors should begin obtaining DNA analysis of both normal and cancer tissue from patients. That would allow doctors to screen out normal genetic variations and more accurately determine the causes of a person’s cancer. 

“Of course we all want precision medicine, but one conclusion from these analyses is that we cannot have precision medicine without precision genomics,” Velculescu said. “We can’t expect physicians to provide the right therapy to the right patients if we can’t obtain accurate results in our diagnostic tests.”


  1. Sian, J, et al. Sci. Transl. Med, April 2015; doi: 10.1126/scitranslmed.aaa7161.