WASHINGTON, (Reuters) – Tumors can not only spread  through the body by sending out tiny cells called seeds, but  they can re-seed themselves, researchers said in a report yesterday that may help explain why tumors grow back even after  they are removed.

They said their findings, published in the journal Cell,  may also help lead to the development of new drugs to stop the  process of cancer spread, or metastasis.

“Circulating tumor cells can also colonize their tumors of  origin, in a process that we call ‘tumor self-seeding’,” Joan  Massague of the Memorial Sloan-Kettering Cancer Center in New  York and colleagues wrote.

“Now we have found that tumors can recapture some of their  most delinquent children, enriching themselves with the most  aggressive metastatic cells, enabling them to grow faster and  more robustly,” Massague, a Howard Hughes Medical Institute  researcher, said in a statement.

“Now we are thinking that in some cases, maybe treatment  left inflamed tissue that had been a home for those cells that  escaped and were residing somewhere temporarily, perhaps in the  bone marrow,” he added.

“They may have re-entered the circulation in the weeks and  months after surgery, and now, through the self-seeding  process, have homed in on this tissue and reproduced the  tumor.”

Massague’s team used mice, injecting them with human breast  cancer cells that had been genetically engineered with a  jellyfish protein to make them glow green under ultraviolet  light.

They tracked these cells as they spread through the bodies  of the mice.

Immune system signaling chemicals, including interleukin 6  and interleukin 8, appear to “call” the tumor cells home,  Massague’s team found.

Researchers are working on cancer vaccines that could  harness the immune system to attack cancer cells more  effectively.

This study suggests it might also be necessary to  tone down some aspects of the immune system.