WASHINGTON (Reuters) – Tumours 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 last week that may help explain why tumours 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 tumour cells can also colonise their tumours of origin, in a process that we call 'tumour self-seeding'," Joan Massague of the Memorial Sloan-Kettering Cancer Center in New York and colleagues wrote.

"Now we have found that tumours 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 tumour."

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 tumour 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.