The types of cancer affecting children, as well as what causes cancer in children, are quite different to those affecting adults.
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.
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.
There has been much concern about the possible health effects of the electric and magnetic fields associated with both power lines and electrical sources in the home.
Since the first study reporting an association between childhood cancer and electrical wiring was published, at least 25 further studies have been published including three major pooled analyses, all three of which were supportive of a link.
The largest single study in England and Wales to date reported a significantly increased risk of leukaemia in children living within 600 metres of a high voltage overhead power line.
The epidemiological evidence has led to the International Agency for Research on Cancer (IARC) to classify extremely low-frequency electric and magnetic fields (ELF EMF) as ‘possibly carcinogenic’ to humans.
Research aimed at finding a biological mechanism for the health effects of EMF is ongoing.
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.
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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