Several types of cancer are virtually unique to children, but the cancers most often seen in adults – including those of the lung, breast and stomach – are extremely rare in children. Most types of cancer become more common as we get older.
The changes that make a cell become cancerous take a long time to develop. There have to be a number of changes to the genes within a cell – these can happen by accident when the cell is dividing, or they can happen because the cell has been damaged by carcinogens. The damage is then passed on to ‘daughter’ cells when the cell divides.
The longer we live, the more time there is for these genetic mistakes to occur. Children – and especially infants – have had little time to acquire these mistakes.
There need to be a number of genetic mutations within a cell before it becomes cancerous. Sometimes a person is born with one of these mutations already present. This doesn’t mean that they will definitely get cancer, but it makes it more likely. This is called ‘genetic predisposition’.
This genetic predisposition may either be inherited or the result of a genetic mutation which occurs when the child is in the womb:
The vast majority (90%) of children born with the faulty Rb1 gene develop retinoblastoma. The picture foris very different in that for every child who develops leukaemia – around 100 have the mutation but don’t develop the disease.
In most children born with a genetic predisposition, whether inherited or acquired, it appears that a further trigger is required for progression to overt disease.
In the case of childhood leukaemia, the ‘two-hit hypothesis’ proposes that initiating events take place whilst the child is still in the womb. The second ‘hit’ occurs later in life, triggering the development of full-blown leukaemia.
Scientists are trying to establish what form these ‘hits’ take – i.e. what factors (other than spontaneous error) cause the initial genetic mutations and what factors trigger progression of the disease.
Despite a wealth of research, much uncertainty remains over what causes cancer in children.
Many different factors have been linked with the development of childhood cancer, with varying degrees of certainty.
Research is complicated by the fact that there are many different factors which may cause cancer in children. Exposure to more than one of these factors is probably necessary – and probably at different stages of a child’s life.
The relative rarity of childhood cancers further impedes research.
Leukaemia is better represented in research literature than other forms of cancer because it affects more children, making it easier to obtain meaningful results in epidemiological studies. International collaborations are important as they increase the number of cancer cases available for study.
The link between childhood cancer and ionising radiation is well-established. Some of the first evidence came from a study of children whose mothers received abdominal X-rays during pregnancy, a practice which is now avoided.
Further evidence comes from studies of the Japanese atomic bomb survivors. There has been a general excess of cancer in the exposed population, but the greatest excess was for leukaemia. The risk for those exposed at young ages was especially pronounced.
The Chernobyl nuclear reactor disaster in 1986 resulted in a marked excess of thyroid cancer in children in the vicinity of Chernobyl. The excess, which began to appear five years after the incident, is apparent in children who were less than 10 years of age at the time of exposure.
Reports during the 1980s and 90s of excesses of childhood leukaemia in the vicinity of certain nuclear installations (around Sellafield in England, Dounreay in Scotland and La Hague in France) caused a great deal of concern.
Radiation doses from exposure to discharged radioactivity, however, were considered to be too low to account for the cluster. The most favoured explanation for these – and other – clusters relates to unusual patterns of population mixing (see “Infections” below).
It has been suggested that exposure to natural background radiation, to which we are all constantly exposed, may be a factor in the aetiology of childhood cancer, particularly leukaemia.
Although not proven, published risk estimates suggest that 15-20% of childhood leukaemia cases in Great Britain could be attributable to this exposure.
From powerlines to mobile phones: electric fields, magnetic fields, electromagnetic fields and electromagnetic radiation
The electricity supply generates electric and magnetic fields. Electric fields come from the voltage on cables and magnetic fields result from the current flowing through them. This means that we are all exposed to both electric and magnetic fields, in the home or workplace. There has been considerable research into the potential health consequences of exposure to these fields, although most of research has concerned the magnetic fields.
Magnetic field levels to which we are normally exposed are measured in units called microtesla, written as µT. Average levels in the home are generally low at around 0.05 µT. Levels are higher near appliances such as hair driers, but we tend only to be exposed for short periods. Magnetic fields near high voltage powerlines can extend up to 200 metres away with levels of several or even tens of µT directly underneath.
In the last 20 years, evidence has accumulated that continuous exposure to magnetic fields increases the risk of childhood leukaemia, findings which in 2002, led the International Agency for Research on Cancer, IARC, to class Extremely Low Frequency Magnetic Fields, ELF-MFs as a possible carcinogen.
The most recent pooled analysis of international studies found a 2.4-fold increased risk of acute lymphoblastic leukaemia for average magnetic field exposures above 0.4 µT and a 1.3-fold increased risk above 0.2 µT. While these levels are higher than the average generally found in the home, they are below those found near high voltage overhead powerlines.
There is some laboratory evidence that magnetic fields can damage DNA – a feature common to known cancer causing agents such as ionising radiation and carcinogenic chemicals.
Magnetic fields from MRI scanners
Magnetic Resonance Imaging or MRI relies on magnetic fields to very accurately image the body. Importantly it does not use x-rays. In MRI scanning exposure to magnetic fields is very short and there are no known side effects of the magnetic field exposure in an MRI scan.
Electromagnetic radiation from mobile phones
Mobile phones are radio transmitters and receivers. Thus, they have electric and magnetic field associated with them. They also emit electromagnetic fields in the form of radio waves or electromagnetic radiation, EMR.
There is concern that exposure to EMR from mobile phones is harmful to health. Following a review of evidence that long-term use of mobile phones may increase the risk of brain tumours in adults, IARC in 2011 classified Radiofrequency Electromagnetic Fields as possibly carcinogenic to humans.
While there are ongoing studies, a link between mobile phone use and cancer in children has not been established at this time. However, international agencies, including Public Health England have urged precaution to limit children’s exposure to mobile phone EMR.
The ultra-violet component of sunlight is known to increase the risk of skin cancer in adults. Australia and New Zealand have a relatively high incidence of childhood melanoma which may be due to UV radiation.
Scientific references and further reading:
Powerlines, magnetic fields and childhood leukaemia
Mobile phones – use by children
The possible role of infections in childhood cancer has been much-studied.
Viruses are known to be implicated in some human cancers including:
These associations can only account for a tiny proportion of childhood cancer in western countries.
There is a great deal of support for a role of infection in the development of childhood leukaemia. Acute lymphoblastic leukaemia (ALL) has an incidence peak at 2-4 years of age, coinciding with the timing of common childhood infections such as measles. However, despite intensive research efforts, no specific leukaemia-causing virus has been identified in children.
Two different theories suggest that rather than being the result of a specific leukaemia-causing virus, childhood leukaemia could be the result of an abnormal response to a common infection.
A cluster of cases of childhood leukaemia around the nuclear reprocessing plant of Sellafield in Cumbria in the 1980s caused speculation that the cluster must be related to radiation exposure.
However, assessments showed that radiation doses were much too low to account for the cluster.
The isolated village of Seascale, at the centre of the cluster, had seen a rapid influx of families. They were brought together from all over the UK to work at Sellafield, and undoubtedly bringing with them an influx of infections.
This provided a basis for suggesting that some childhood leukaemia clusters might be an unusual outcome of a common infection arising in non-immune individuals following ‘population mixing’.
Although extremely difficult to prove definitively, this theory has been borne out by studies of other leukaemia clusters.
Many types of chemicals are known to be carcinogenic. The question is whether children are exposed to high enough levels to lead to the development of cancer. Exposure in utero may also be important.
It is generally accepted that a healthy diet with plenty of fresh fruit and vegetables can protect against cancer in adults.
At least one study has shown a protective effect in children: Kwan and colleagues demonstrated a strong protective effect against childhood leukaemia risk if oranges and bananas were consumed on a regular basis during the first two years of life.
There is a fairly substantial body of evidence pointing towards a small protective effect of breast-feeding on childhood leukaemia risk.
Cancer is the leading cause of death in children aged 1-14 years in the UK and survivors can face a lifetime of serious health issues as a result of the intensive treatments used to treat their cancer.
In addition to what causes cancer in both children and adults, the cancers themselves are different and dedicated research is needed into each.
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