Apparently "normal" cells may carry cancer to new sites long before a tumour develops, lying dormant until key genes are activated, experts say.
US researchers say their findings, published in Science, could explain why some breast cancers lead to new tumours long after the disease is treated.
Secondary, or metastatic, cancers are responsible for the majority of deaths from the disease.
UK experts said it was essential to know more about how the cancer spreads.
"These are important but early results"
It has traditionally been thought that the spread of cancer to another site in the body was a late event that only occurred when a disease was advanced.
Cancer cells had been thought to have stayed in place until they had undergone a series of genetic alterations making them more aggressive.
They have to be able to survive the journey through the bloodstream and be able to initiate malignant growth in their new environment in the new site.
Lying in wait
In this latest research, by a team at the Memorial Sloan-Kettering Center in New York, mice were injected with normal breast tissue cells which had been manipulated so the scientists could "switch on" cancer genes (oncogenes).
It was found that the cells were capable of travelling in the bloodstream to the lungs and surviving there for up to 16 weeks without expressing any oncogenes.
The cells did not begin growing aggressively in the lungs until the oncogenes had been turned on.
The researchers say that examining each step of the process by which cancer metastasizes, including those involving normal cells, it might be possible to work out ways to destroy the cells responsible for the disease's spread through the body.
Writing in Science the researchers, led by Dr Katrina Podsypanina, said: "The finding that metastatic disease can arise from untransformed mammary cells in the circulation refines our conception of cancer progression."
Liz Baker, senior science information officer for Cancer Research UK, said: "Learning more about the spread of cancer - or metastasis - is essential because it's harder to treat the disease once it has spread.
"These are important but early results in mice - it will be interesting to see whether this can one day help the outcome of people affected by cancer."
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