A new safe and cheaper blood test allows doctors to see how lung cancer patients are responding to treatment.

Lung cancer is one of the most common and serious types of cancer with around 44,500 Britons diagnosed every year.

Overall, about one in three live for at least a year after diagnosis and about one in 20 people live at least 10 years, but survival depends on how far the cancer has spread.

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A new safe and cheaper blood test allows doctors to see how lung cancer patients are responding to treatment

It's common for cancer therapies to fail after a few months, often because the cancer evolves resistance to the treatment.

At this stage its important for doctors to understand how the tumour is changing.

Lead author Dr Seung-min Park, instructor at Stanford University School of Medicine said: 'Without a biopsy and genetic profiling, we are flying blind, trying to select a second or third option for therapy and hoping it works

'Blood-based monitoring would allow us to select the right second and third therapies instead of flying blind.'

A single state-of-the art biopsy of lung tissue with DNA sequencing costs about £14,300 and results can take up to three weeks.

And CT scans to see whether a tumour is shrinking or growing increase the body's exposure to damaging X-rays.

Now Dr Park has come up with a simple blood test that profiles the genes of lung-tumour cells that is fast with results in hours and 600 times cheaper.

It requires no more than 2 millilitres of blood - about half a teaspoon with results available in less than five hours at just £24.

Dr Park said evaluating the genetic profiles of tumour cells circulating in the bloodstream could transform cancer care.

It can indicate the next chemotherapy or targeted therapy to use when tumours evolve resistance to previous drugs and provides a way to study how tumours change over time.

The system isolates circulating tumour cells from the blood of cancer patients and reading a handful of genes from inside each tumour cell.

Researchers were able to obtain genetic information about the original cancer tumour that resides deep in the lungs without doing a biopsy, which can be dangerous for the patient.

Dr Park said: 'We are trying to make minimally invasive technology that allows us to continuously monitor one person's health over time.

Finding a way to look at circulating tumour cells, or CTCs, in the blood has been a goal of oncologists for years.

Cancer kills usually from metastasis, the spread of tumours throughout the body.

Dr Seung-min Park, instructor at Stanford University School of Medicine, came up with the new test, which only requires 2 millimetres of blood

Part of metastasis is the entry of tumour cells into the bloodstream, where they circulate along with normal blood cells, eventually landing in other organs and initiating tumours there.

In general, the presence of CTCs in the blood of cancer patients predicts patients will live for a shorter time.

But blood typically contains very few CTCs and separating them from blood has been a problem.

The new technique involves taking blood from lung cancer patients and then attaching antibodies to circulating tumour cells.

Once the cancer cells are labeled, the team introduces magnetic nanoparticles designed to attach to the antibodies labeling the cancer cells.

With each individual cancer cell labeled with a magnetic nanoparticle, the researchers can then use a device called a magnetic sifter, or MagSifter, to isolate them for genetic analysis.

Co-author Dr Dawson Wong said: 'We validated our device on lung cancer because of the difficulties of doing lung biopsies.

'But the technology is not limited to profiling lung cancer. We could swap out markers and adapt the technique to other types of cancers.'