Researchers working with the charity Cancer Research UK looked at the number of circulating tumour 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.

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)