Children with leukaemia are to be offered a new test, which could transform the way they are treated.
Doctors currently check for leukaemia cells under a microscope
It enables doctors to accurately measure the cancer cells left over after the child has chemotherapy.
Doctors can then decide whether or not they need more treatment and if they are at risk of relapse.
Until now they have had to examine blood or bone marrow samples under a microscope, to try to identify leukaemia cells.
This has not proved very accurate and, as a result, some children with residual cells are missed while others with no sign of the disease have chemotherapy unnecessarily.
Every child who is treated for acute lymphoblastic leukaemia (ALL) - the form of the disease most commonly found in children - will still have cancer cells in their bone marrow after a month of chemotherapy, what doctors call minimal residual disease or MRD.
Levels can range from between one in 20 cells to one in 10,000 cells. Those with higher levels are most likely to suffer a relapse.
The new technique involves using a molecular microscope to detect the number of cancer cells left over in the bone marrow.
It will be used on children who develop leukaemia in the UK over the next six years as part of a major clinical trial. It follows successful pilot studies.
The trial will look at whether the results from the test can be used to tailor-make treatments to suit the individual needs of the patient.
"Until now the only way to monitor a child's response to treatment has been to look down a microscope for leukaemia cells in blood or bone marrow samples," said Dr Nick Goulden, a consultant who works at Bristol Royal Hospital for Children.
"After a decade of refining this 'molecular microscope' and a crucial year-long Leukaemia Research Fund pilot study we are now in a position to exploit the amazing potential of this test," he said.
"This test will enable us to select which patients are at high risk of relapse and which children could actually be cured with significantly less chemotherapy," he said.
"For a child with low risk MRD we will be able to reduce the amount of time they spend in hospital by up to 60%.
"This would represent a major improvement in quality of life - and a step closer to our goal - cure with the minimum of disruption."
Sir Walter Bodmer of the Leukaemia Research Fund, which is contributing towards the cost of the trial, welcomed the study.
"This is an exciting time for leukaemia medicine as the power of molecular biology allows doctors to treat children with leukaemia in a far more informed controlled way, whilst at the same time striving to reach that final goal of 100% survival."
Each year, 450 children are diagnosed with acute
lymphoblastic leukaemia - approximately 75% of all cases of childhood leukaemia.
Massive improvements in treatment have been made over the last 30 years and survival for childhood ALL is now approaching 80%.