WASHINGTON (Reuters) - Treating cancer with surgery, chemotherapy or radiation may sometimes cause tumors to spread and U.S. researchers said on Thursday they may have nailed down one of the causes -- a compound called TGF-beta.

Tests in mice show that using the chemotherapy drug doxorubicin or radiation both raised levels of TGF-beta, which in turn helped breast cancer tumors spread to the lung.

But using an antibody to block TGF-beta stopped the process, Dr. Carlos Arteaga and colleagues at Vanderbilt University in Tennessee reported.

Developing drugs that block TGF-beta might help prevent cancer from recurring, Arteaga's team reports in the May issue of the Journal of Clinical Investigation.

"The repopulation and progression of tumors after anti-cancer therapy is a well-recognized phenomenon," the researchers wrote. "It has been shown to occur following radiotherapy, chemotherapy, and surgery."

Cancer experts have wondered if the so-called primary tumor -- the first and biggest tumor -- might somehow suppress the growth of other tumors, and that removing or destroying the first tumor might allow other, undetectable, tumors to then grow.

TGF-beta, which is involved in both the growth and suppression of tumors, may hold part of the answer, Arteaga's team said...