A phase three clinical trial has revealed simple blood test can help doctors decide the best way to treat a man with advanced prostate cancer. According to the study, a circulating tumor cell or CTC count can predict which men are likely to respond to standard treatment and live longest and can benefit from more aggressive new drug trials.

CTCs are tumour cells that have broken off from a primary tumour and are carried around the body in the blood. They can be a potential indicator of early cancer metastasis and can precede a clinical cancer diagnosis. Researchers from the USC Norris Comprehensive Cancer Center in California said they have been looked at before for prostate cancer but are found only in later stages.

"No one, until now, has looked at whether CTC counts can be used right at the beginning, when a man first presents with metastatic prostate cancer, to tell us whether he's going to live a long time or short time, or whether or not he will progress with therapies," said lead study author Dr. Amir Goldkorn, associate director of translational sciences at USC Norris Comprehensive Cancer Center in California.

Dr. Goldkorn’s team found that men with higher blood levels of CTCs had shorter median survival times and a greater risk of death during the study period. Their disease could only be controlled by treatment without getting worse for a shorter time. "You couldn't tell these men apart when they walked in the door," Goldkorn said in a USC news release. "All of their other variables and prognostic factors were seemingly the same, and yet they had very, very different outcomes over time."

For the study, part of a phase 3 clinical trial of the SWOG Cancer Research Network, researchers said they used CellSearch – an exclusive and only actionable blood test used for detecting CTCs in cancer patients with metastatic breast, prostate, or colorectal cancer.

The study analyzed blood samples from 503 men with metastatic prostate cancer who were participating in the drug trial. The study said patients with five or more CTCs in their blood sample had the poorest outcomes. They were more than three times as likely to die during the study period than patients with no CTCs, and nearly 2.5 times more likely to see their cancer progress.

They were also more likely to have a poor response to treatment, as measured by a complete prostate-specific antigen (PSA) response. Men with five or more CTCs lived a median of 27.9 months after their blood test, compared to 56.2 months for men with one to four CTCs and at least 78 months for men with none. (Half lived longer, half for a shorter time.)

Researchers say more CTCs mean faster cancer progression, poorer response to standard treatment, and shorter survival time. The findings that were published in the journal JAMA Network Open, show that measuring CTC levels when therapy begins can predict long-term survival.

"We want to enrich these clinical trials with men who need all that extra help -- who really would benefit from three drugs versus just two, or from being on a new chemotherapy drug, even though it may have more side effects," Goldkorn said. His team is now testing a new blood test that also looks at the molecular makeup of CTCs and tumor DNA in the blood, along with other factors. Their aim is to identify biomarkers that may be even better predictors of prognosis.

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