If used routinely, this blood test could reveal when treatment for advanced prostate cancer stops working and actually begins promoting tumor growth, the researchers suggested.

"Our study showed that a steroid treatment given to patients with advanced prostate cancer and often initially very effective started to activate harmful mutations and coincided with the cancer starting to grow again," study leader Dr. Gerhardt Attard, from the Institute of Cancer Research (ICR) in London, explained in an ICR news release.

"In the future, we hope to routinely monitor genetic mutations in patients with advanced disease using just a blood test -- enabling us to stop treatments when they become disease drivers and select the next best treatment option. We need to confirm these findings in larger numbers of patients, but using these types of blood tests could allow true personalization of treatment for prostate cancer patients, based on the cancer mutations we detect," he explained.

Using a blood test to measure circulating tumor DNA levels is less expensive and less invasive than needle biopsies. This test could be an effective way to monitor the emergence of treatment-resistant prostate cancer, the study published on Sept. 17 in Science Translational Medicine suggested.

"Drug resistance is the single biggest challenge we face in cancer research and treatment, and we are just beginning to understand how its development is driven by evolutionary pressures on tumors," Paul Workman, interim chief executive at the ICR, said in the news release.

This discovery "reveals how some cancer treatments can actually favor the survival of the nastiest cancer cells, and sets out the rationale for repeated monitoring of patients using blood tests, in order to track and intervene in the evolution of their cancers," Workman said.

"There are currently too few treatment options for men living with advanced stage prostate cancer. Not only do we desperately need to find more treatments for this group of men, we also need to understand more about when those that are available stop working and why," Dr. Matthew Hobbs, deputy director of research at Prostate Cancer UK, said in the news release.

"This research is important as it shows that there might be a new way to monitor how a man's cancer is changing during treatment, and that could help us to pinpoint the stage at which some drugs stop being effective. In the future, this could arm doctors with the knowledge they need to ensure that no time is wasted between a drug that stops working for a man and him moving on to another effective treatment," Hobbs said.

But, Hobbs also noted that this is preliminary research and that the study size was small -- just 16 men. He agreed with Attard that the findings need to be confirmed in a larger study.

The researchers cautioned that any patients currently taking medication for advanced prostate cancer should continue to take their medications as prescribed and discuss any concerns about their treatment with their doctor.