British scientists have found that counting the number of lung cancer cells circulating in a patient's blood could help determine how aggressive the cancer is and predict the best treatment to use.

Researchers working with the charity Cancer Research UK looked at the number of circulating tumor cells, or CTCs in blood samples of 101 patients with a type of the disease called non small cell lung cancer before and after they had undergone one cycle of chemotherapy.

They found lung cancer patients with five or more CTCs had a significantly worse survival rates. The average overall survival was 4.3 months for patients with five or more CTCs compared to 8.1 months for patients with fewer than five.

The findings suggest that counting CTCs could be a simple way to monitor how well a patient is responding to treatment within a few weeks of starting it, the researchers said.

And being able to detect when CTC numbers are rising could give doctors the option to move patients on to new treatments more promptly.

"We now need to test our findings in more patients but, if our results are confirmed, there is now the potential to tailor treatments to individual patients and find new ways to treat the disease," said Fiona Blackhall, a doctor from The Christie cancer hospital in Manchester who worked on the study.

Lung cancer kills 1.2 million people a year around the world and is one of the lowest survival rates of any cancer because over two-thirds of patients are diagnosed at a late stage when curative treatment is not possible.

More than 80 percent of lung cancers are caused by smoking, and less than 15 percent of people diagnosed with the disease survive longer than five years, according to the World Health Organization (WHO). In Britain, lung cancer is the second most common cancer and was diagnosed in around 41,000 people in 2008.

Chemotherapy and radiotherapy can slow the growth and spread of lung tumors, but in most patients the cancer returns and is also generally more resistant to treatment.

Blackhall's team, whose study was published in the Journal of Clinical Oncology, noted there are no tests available that provide early warning about resistance, but said they hoped their findings might change that.

"In the near future we hope to be able to use gene sequencing tools to learn more about CTCs," said Caroline Dive of the University of Manchester, who also worked on the study.

"If we can do this before a patient has chemotherapy and then again later if the cancer returns, we may be able to learn more about the processes that lead to drug resistance, and ultimately develop new drugs."