The types of cancer affecting 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.

  • What causes cancer in childhood?

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.

  • The genetic basis of childhood cancers

There need to be a number of genetic mutations within a cell before it becomes cancerous.

Genetic predisposition may either be inherited or the result of a genetic mutation which occurs when the child is in the womb.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:

Retinoblastoma is an example of a type of cancer which is known to be caused by an inherited faulty gene in some children (two out of five cases are inherited).

Children born with a genetic predisposition to leukaemia, on the other hand, are known to acquire the predisposing genetic mutation whilst they are in the womb.

The vast majority (90%) of children born with the faulty Rb1 gene develop retinoblastoma.

The picture for leukaemia is very different in that for every child who develops leukaemia – around 100 have the mutation but don’t develop the disease.

  • Infections

The possible role of infections in childhood cancer has been much-studied.

Viruses are known to be implicated in some human cancers including:

Burkitt lymphoma
Hodgkin lymphoma and nasopharyngeal carcinoma (all associated with Epstein-Barr virus)
liver carcinoma (hepatitis B) and
Kaposi sarcoma (HIV and HHV8)

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 two to four 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.

The ‘delayed infection’ hypothesis suggests that a lack of exposure to infection early in life results in a poorly primed immune system which reacts inappropriately when the child is subsequently exposed to infection.

The ‘population mixing’ hypothesis also suggests that leukaemia is a rare response to a common infection in susceptible children.

  • Chemical exposure

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.

Chemicals contained in air pollution – including benzene – are known to be carcinogenic. The levels to which most children are exposed are extremely low and the risks are difficult to detect because exposure is ubiquitous. There is some evidence of an increased risk of childhood cancer for children with birth addresses within 1km of hot spots for various air pollutants.

  • Associations have also been reported with exposure to pesticides.

The evidence for a link between childhood cancer and parental smoking is mixed. A meta-analysis of more than 30 studies showed a 10 per cent increase in risk of all cancers with maternal smoking during pregnancy, but no evidence for an increased risk of any specific cancer. At least one study has found increased chromosomal abnormalities in amniocyte cells of foetuses of smoking mothers.

Some types of medication and drugs have been linked to childhood cancer. There have been reports of the possible carcinogenic effects of many different drugs taken by mothers during pregnancy. The only one firmly established as a transplacental carcinogen is diethylstilboestrol (DES), a hormone which in some countries used to be given to pregnant women to prevent miscarriage but which has now been discontinued. DES caused an unusual type of cancer in girls and young women.

Some of the drugs used in chemotherapy are known to carry a risk of causing secondary cancer. A major aim of ongoing research is to develop drugs which are less toxic.

  • Protective factors

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.

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