Cancers use cells in bone marrow to create a fertile environment for tumors to spread to other parts of the body, according to a study of mice.

The bone marrow cells help make blood vessels that provide oxygen and other nutrients needed by spreading cancers, said researchers led by Rosandra Kaplan, an oncologist at Cornell University's Weill Medical College in New York. The study appears in today's issue of Nature.

The experiments on mice with lung cancer seem to answer longstanding questions about what gives tumors the power to invade new tissues. Drugs such as Avastin from Genentech Inc. and Roche Holding AG that can slow blood vessel growth in existing cancers also might help doctors prevent metastasis, wrote Patricia Steeg, director of molecular therapeutics at the US National Cancer Institute in Bethesda, Md., in a commentary accompanying the study.

''This article is suggesting we need to use these drugs much earlier" to prevent cancer from spreading, Steeg said in a telephone interview yesterday. Bone marrow cells are ''setting up a pre-metastatic site we didn't know existed."