Chemotherapy can backfire by helping healthy body cells to fuel treatment-resistant cancer and growth, research has shown.
The surprise discovery suggests that many forms of cancer treatment can actually make the disease tougher to tackle.
Almost all solid tumour cancers, such as those affecting the breast, prostate, lung and bowel, ultimately stop responding to chemotherapy.
Scientists now believe one reason why could be the effect the treatment has on healthy connective tissue cells called fibroblasts.
DNA-damaging cancer drugs cause fibroblasts to pump out growth molecules into the tumour micro-environment belonging to the WNT family of proteins, the study found.
High levels of one called WNT16B helps cancer cells to grow, invade surrounding tissue, and resist chemo treatment.
Working in the laboratory, scientists observed an up to 30-fold increase in WNT production in response to chemotherapy.
"Cancer cells inside the body live in a very complex environment or neighbourhood," said lead scientist Dr Peter Nelson, from the Fred Hutchinson Cancer Research Centre in Seattle, US.
"Where the tumour cell resides and who its neighbours are influence its response and resistance to therapy."
Blocking the treatment response of fibroblasts could improve the effectiveness of chemotherapy, say the scientists whose findings are reported in the journal Nature Medicine.
The team examined cancer cells from prostate, breast and ovarian cancer patients who had been treated with chemotherapy.