WASHINGTON, Aug. 28 (Xinhua) — The spread of cancer to new sites in the body — the process that is ultimately responsible for most cancer deaths — may happen earlier in the disease process than was previously thought, a new research on mice suggests.

    The findings by U.S. researchers from Memorial Sloan-Kettering Cancer Center appears in the Aug. 29 issue of the journal Science.

    This discovery suggests an explanation of why some breast cancers, for example, metastasize long after the initial tumor has been treated, said the journal.

    Cancer metastasis involves multiple steps. The cells must be equipped to survive the trip in the bloodstream and initiate malignant growth in their new environment. So researchers have traditionally considered the metastasis to be a late event in cancer progression that occurs after primary tumor cells have racked up a series of genetic alterations that switch on cancer genes.

    The new results suggest that metastatic disease might instead arise from normal cells that spread relatively early in the disease and remain dormant at the new organ site until cancer genes are switched on.

    The research team injected mice with normal mammary cells that had been experimentally manipulated in a way that allowed the researchers to turn on certain cancer genes, or "oncogenes," at various times after injection.

    They found that the normal mammary cells were capable of traveling 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.

    Examining each step of the process by which cancer metastasizes, including those involving normal cells, might allow scientists to identify new strategies for destroying the cells responsible for the disease’s spread through the body, the authors suggest.