How Cancer Spreads

For the past four years, Pat McWaters has been in the fight of her life against a cancer that seems to be moving constantly, CBS News medical correspondent John LaPook reports.

"Breast, liver, spine, sternum, ribs, pelvis ... all cancer," Pat says.

She's had a variety of treatments to try and control her disease, which began in her left breast. Pat's case is an example of a patient's greatest fear: the spread of cancer, or metastasis.

Breast cancer is not fatal if confined to the breast. But when the cancer cells travel to a vital organ like the liver, they can overwhelm the liver cells and shut it down. Ninety percent of cancer deaths occur because of where cancer ends up, not where it begins.

"The common belief is that metastasis is fatal. That is incorrect," says Dr. Josh Fidler.

Fidler has spent his whole research life trying to figure out how cancer spreads. He sees cancer cells as seeds looking for soil. In all, 99.99 percent of the cells that spread from the original tumors die. Only the strongest survive, and they need fertile soil to grow. Several new cancer therapies try to ruin the soil by cutting off blood flow to the tumors.

Because it takes just one cell for cancer to spread, patients are often treated with chemotherapy even though their original tumor is removed. Doctors are looking to destroy cells they can't yet see.

"It takes many, many months and years to grow to a size that we can diagnose," Fidler says.

The next step to stop cancer: Take a piece of a patient's tumor, analyze it to see if it's likely to spread, then tailor an effective treatment.