Chemotherapy could cause cancer to spread and become more deadly, new research claims.
The treatment is often regarded as the first option for breast cancer patients to shrink tumors and even blitz the disease altogether.
However, scientists at New York's Albert Einstein College of Medicine have found evidence that this is only a short-term solution.
Their research suggests that, while shrinking the tumors, chemotherapy simultaneously opens a gateway for tumors to spread into the blood system, making it easier to grow back stronger.
Cancer becomes incredibly difficult to treat--often fatal--once it spreads to other organs; it is then classified as Stage 4.
Lead author Dr George Karagiannis says the findings, published on Wednesday, should not deter patients from seeking treatment, but suggests we could create a way to better monitor tumor movement in patients undergoing chemotherapy.
'One approach would be to obtain a small amount of tumor tissue after a few doses of preoperative chemotherapy,' Dr Karagiannis told the Telegraph.
'If we observe that the markers scores are increased we would recommend discontinuing chemo and having surgery first, followed by post-operative chemo.'
Chemotherapy can be administered as a pill or through an intra-venous drip.
The drugs travel throughout the body in the bloodstream.
It is tipped as an effective way to reach cancer cells that may have spread away from the tumor, as is common with breast cancer.
However, this is not the first study to demonstrate the ways in which chemotherapy can trigger secondary cancers. A 2012 study by the Fred Hutchinson Cancer Research Center in Seattle found chemotherapy activates healthy cells to fuel tumor growth.
Senior author Peter Nelson, a human biology professor, said that in theory chemotherapy is perfect for killing cancer cells - mix a toxic dose of the drug with a tumor in a lab dish, and the tumor will not be able to survive or spread.
However, he said, the dose required to kill tumors is lethal for patients.
As such, doctors have to administer a lower dose, and it seems that has two key downsides. First, it facilitates dangerous spreading. Second, it allows some tumor cells to survive, become resistant to chemotherapy, and spread to other organs.
Secondary cancer is also referred to as metastatic cancer. It means the cancer has spread from the original site to another organ, via the lymph nodes.
This is also called Stage 4 cancer, the last stage. It can be incredibly difficult to treat as the tumors tend to be more aggressive and resistant to treatment. In many cases the tumors may have also lodged themselves in delicate areas that can be dangerous to operate on.
'We are currently planning more extensive trials to address the issue,' Dr Karagiannas said.
'In this study we only investigated chemotherapy-induced cancer cell dissemination in breast cancer. We are currently working on other types of cancer to see if similar effects are elicited.'