Leukaemic cancer cells left in the body after treatment are referred to as Minimal Residual Disease (MRD) and are the major cause of relapse.
When a child is first diagnosed with leukaemia they may have as many as one million million leukaemic cells in their blood and bone marrow. This level of disease can be easily detected by examining a blood or bone marrow sample under a microscope.
The aim of the first stage of treatment is to reduce the burden of leukaemic cells to a level which allows production of normal blood cells to resume. A child is said to be in remission when they reach this point.
However a child in remission may still have as many as one thousand million leukaemic cells in their body. This is known as minimal residual disease (MRD). This level of disease cannot be readily detected under the microscope.
Every child is given further treatment in order to completely eradicate the disease from their body.
It is important to know how much leukaemia is still present in a child’s blood and bone marrow at remission because this helps to predict how likely it is that the child will relapse.
Until recently doctors did not have an accurate method for measuring MRD. But many years of research have culminated in the development of a breakthrough molecular technique for measuring MRD.
Use of this new MRD test formed the basis of the most recent national clinical trial for childhood ALL – UK ALL 2003. Doctors used the test to assess relapse risk in more than 2,500 children undergoing treatment for ALL. This enabled them to tailor each child’s treatment to their individual needs.
The aim was not only to improve the chance of cure in high-risk children but to reduce toxicity in low-risk children by not giving them more treatment than they needed.
As Dr Nick Goulden, Consultant Paediatric Haematologist at Great Ormond Street Hospital, explains: “When you’re treating children, the most important thing is to do as little harm as possible. Chemotherapy does have side effects – both in the short-term and in the long-term. Anything that allows us to get the same cure rates but reduce toxicity has to be a good thing.”
The trial was so successful that the NHS has now adopted MRD testing as part of the standard treatment regime for children with ALL. It is thought that this innovation may drive the survival rate for childhood ALL above 90%.
Children with Cancer UK invested more than £3 million in this ground-breaking work. The ongoing costs of MRD testing will now be borne by the NHS.