Cancer tumours may have a head start of 10 years or more before available blood tests can detect them, a study has found.
Scientists came to the conclusion after analysing the development of ovarian cancer. They believe the findings are broadly applicable to all solid tumour types, such as those responsible for breast, prostate and lung cancer.
"The study's results can be viewed as both bad and good news," said lead researcher Professor Sanjiv Gambhir, from Sanford University in California, US.
The bad news was that by the time a tumour reaches detectable size cancer cells are likely to have spread to other parts of the body. Cancer spread, or metastasis, is chiefly responsible for deaths from the disease.
"The good news is that we have, potentially, 10 or even 20 years to find the tumour before it reaches this size, if only we can improve our blood-based methods of detecting tumours," Prof Gambhir added.
The study, published in the journal Science Translational Medicine, is based on a mathematical predictive model originally used to predict concentrations of drugs injected into the blood stream.
Prof Gambhir's team applied the same model to biomarkers secreted into the bloodstream by tumours. Blood tests are designed to detect these biomarkers. Two of the best known are PSA (prostate specific antigen) for prostate cancer and CA125 for ovarian cancer. Both proteins are produced in smaller amounts by healthy tissue, complicating the detection of cancer.
Focusing on CA125, the scientists found that before an ovarian tumour could reliably be detected, it would need to consist of about 1.7 billion cells - equivalent to the volume of a two centimetre-wide cube. At typical tumour growth rates, it would take the tumour 10.1 to 12.6 years to reach this size.
The technique could be used to validate new tumour biomarker tests, said Prof Gambhir. "This model could take some of the guesswork out of it," he said.
"It can be applied to all kinds of solid cancers and prospective biomarkers as long as we have enough data on, for instance, how much of it a tumour cell secretes per hour, how long the biomarker can circulate before it's degraded and how quickly tumour cells divide."