Chemotherapy can backfire by triggering healthy cells to secrete a protein that sustains tumor growth, which could explain why some patients become resistant, a new study suggests.

Scientists made the "completely unexpected" finding while assessing why cancer cells are dramatically harder to kill in the body than in a laboratory environment.

Cancer-busting chemotherapy works by stopping the reproduction of fast-dividing cells like ones found in malignant tumors.

About 90 percent of patients with solid cancers like breast, prostate, lung and colon cancers or other metastatic diseases that spread end up developing resistance to treatment.

Chemotherapy is often given at intervals so that the body is not overwhelmed by its toxicity, but experts say that breaks in treatments provides time for tumor cells to recover and develop mutations that boost their survival and help them resist treatment.

Researchers at the Fred Hutchinson Cancer Research Center in Seattle examined fibroblast cells that are essential to wound healing and the production of collagen, the main component of connective tissue like tendons.

The study, published in Nature Medicine, tested the effects of a type of chemotherapy on tissue samples of men with prostate cancer and found "evidence of DNA damage" in healthy cells after treatment by causing fibroblasts or wound-healing cells around tumors to produce up to 30 times more of a protein called WNT16B than they should, fueling cancer cells to grow, invading surrounding tissue and helping tumors resist subsequent chemotherapy.

Scientists already know that the protein was involved in the development of cancers, but the new study is the first to show that the protein was also involved in treatment resistance.

They hope the latest findings will help scientists discover a way to stop this backfire response and improve the efficacy of chemotherapy.

"Cancer therapies are increasingly evolving to be very specific, targeting key molecular engines that drive the cancer rather than more generic vulnerabilities, such as damaging DNA," lead researcher Peter Nelson said in a statement. "Our findings indicate that the tumor microenvironment also can influence the success or failure of these more precise therapies."

The latest research supports previous research that cancer treatments don't just affect cancer cells, but can also affects cells in and around tumors. Experts say that is reaction is not always bad because chemotherapy can help stimulate surrounding healthy immune cells to attack malignant tumors.