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In a "completely unexpected" finding, new research has revealed that chemotherapy can cause damage to healthy cells which triggers them to secrete a protein that sustains tumor growth and resistance to further treatment.

Chemotherapy is a drug treatment that uses powerful chemicals to kill fast-growing cells found in tumors in the body.

Around 90% of patients with solid cancers, such as breast, prostate, lung and colon that spread - metastatic disease - develop resistance to chemotherapy.

Researchers tested the effects of a type of chemotherapy on tissue collected from men with prostate cancer, and found "evidence of DNA damage" in healthy cells after treatment.

The results of the study are published in the journal Nature Medicine.

Researchers at the Fred Hutchinson Cancer Research Center in Seattle examined fibroblast cells. Fibroblast cells normally play a critical role in wound healing and the production of collagen, the main component of connective tissue such as tendons.

Researchers found that chemotherapy causes DNA damage that causes the fibroblasts to produce up to 30 times more of a protein called WNT16B than they should and the protein fuels cancer cells to grow and invade surrounding tissue - and to resist chemotherapy.

Lead researcher Peter Nelson said the findings indicate that the tumor microenvironment also can influence the success or failure of these more precise therapies.

"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," Nelson said.

A cancer research expert from the UK Fran Balkwill, told BBC News that this work fits with other research showing that cancer treatments don't just affect cancer cells, but can also target cells in and around tumors.

"Sometimes this can be good - for instance, chemotherapy can stimulate surrounding healthy immune cells to attack tumors," Balkwill said. "But this work confirms that healthy cells surrounding the tumour can also help the tumour to become resistant to treatment. The next step is to find ways to target these resistance mechanisms to help make chemotherapy more effective."