Millions of people carry tumours which go undetected for a decade because they do not shed tell-tale substances into bloodstream, a new study claims.
The study is the first to connect the size of a tumour with the level of biomarkers it releases into the bloodstream.
Currently available blood tests help doctors identify cancer by the presence of substances which are produced by tumours and seep into the bloodstream.
But a study by Stanford University scientists in America indicates that it could take a cancer cell a decade or even longer to grow into a tumour and begin secreting enough of these so-called biomarkers to be picked up in a screening.
Many biomarkers made by tumour cells are also produced naturally in a healthy body, so it can take years before they reach a high enough concentration to cause concern.
More sensitive tests are needed and more distinctive biomarkers must be found in order to bring about earlier diagnoses of cancer, which are key to the success of any treatment, researchers said.
The study found that an ovarian tumour would have to reach the size of 1.7 billion cells before the best available test could detect it.
It would be the size of a 2cm cube and would have been present in the body for between 10 and 13 years before high levels of its most distinctive biomarker became unusually high, they said.
In theory, a similar biomarker which was not produced elsewhere in the body could be detected within eight years, when the tumour would be the size of a four millimetre cube.
Prof Sanjiv Gambhir, who led the study, said: "It's really important for us to find biomarkers that are made exclusively by tumour cells.
"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."