The debate over the question of escalating childhood cancer rates is currently unresolved.

Childhood cancers typically occur more often in deeper body tissues, such as brain, bone, and lymph glands, as opposed to cancers of the skin and internal organs found more commonly in adults.

Eexperimental and epidemiological evidence suggests that exposures in utero and during early childhood increase the likelihood of cancer occurrence in both children and adults.

Childhood Cancer

Our understanding of the factors causing childhood cancer is even less well defined compared to that for adults. There are several issues that cancer researchers and epidemiologists identify as important missing pieces of the puzzle.

Statistics on Childhood Cancer

The first gap concerns the data on long-term patterns and trends in childhood cancer in this country. Each year, the National Cancer Institute of Canada publishes statistics on childhood cancer for the most recent five-year period available. In 2001, data published for 1992-96 indicate that the age-standardized incidence rate (ASIR) for all childhood cancers was about 160 per million children (ages 0 to 19).8 This represents an average of 1,266 Canadian children diagnosed annually with cancer during that time.9 Leukemia accounts for the greatest proportion of new cancer cases (26%), followed by brain and spinal cord cancers (17%) and lymphomas (16%).10

Some US sources state that pediatric cancer incidence has increased by about 1% per year over the last 25 years.11,12 Certain types of childhood cancers in particular appear to have increased, namely, acute lymphoid leukemia, tumours of the CNS and bone tumours.13 Others, however, suggest that while there was an apparent upsurge in the 1980s due to improved detection (e.g., use of MRI versus CT scans) and classification of tumours, childhood cancer rates have stabilized since the early 1990s.14 This debate is currently unresolved.15

In Canada, we are hampered by problematic statistics on national cancer incidence among children, making it difficult to detect historical trends with any accuracy.16,17 Health Canada's Cancer Surveillance On-line web resource shows that the ten-year (1984 to 1995) incidence rate for childhood cancers (ages 0 to 19 years) hovered around 16 cases per 100,000 or 160 per million, but this snapshot of data is insufficient to prove an increasing trend.

Carcinogenesis in the Young

Secondly, the precise mechanisms by which cancer comes about in children are not yet clear. Childhood cancers are typically of a different variety from those observed in adults.18 They occur more often in deeper body tissues, such as brain, bone, and lymph glands, as opposed to cancers of the skin and internal organs found more commonly in adults. The multistage model for carcinogenesis outlined above does not appear to adequately address all relevant mechanisms and susceptibility factors at different stages of development for children.19

Greater exposure of vulnerable, developing systems to environmental contaminants is one potential factor, and inheritance of damaged germ cell DNA may be another associated with the eventual appearance of cancer in children. There is both experimental and epidemiological evidence suggesting in particular that the former mechanism, that is, exposures in utero and during early childhood, increase the likelihood of cancer occurrence in both children and adults.20

From toxicology we know that tissues with rapid turnover throughout life, such as blood, skin and sperm, and tissues undergoing specific proliferation and terminal differentiation, such as testes or breasts during puberty, are generally more susceptible to the effects of carcinogens. This is because there is less time for DNA repair to occur and DNA is being called upon to carry out its varied functions.21 This fits well with a model of children's greater susceptibility to carcinogens during development.

Surprisingly, the epidemiological evidence for prenatal exposures and childhood cancer is limited. Case-control studies suggest that maternal pesticide exposure during pregnancy is associated with moderate increases in childhood brain tumours and leukemias.22 Maternal consumption of high amounts of cured meat (containing nitroso compounds) during pregnancy has been speculated to increase the risk of childhood brain tumours.23 A recent review of 48 published studies on occupational parental exposures and risk of childhood cancer concluded that the strongest evidence of exposures that increase childhood leukemia is with paternal exposure to solvents, paints and employment in an automobile-related industry.24

In support of the second mechanism of pediatric carcinogenesis (i.e., inheritance of defective germline DNA), there is some evidence that exposures of fathers prior to conception are linked to increased risk of cancer in their offspring. For example, paternal preconceptional exposure to pesticides appears to enhance the risk of childhood leukemia.25 Some studies have similarly attributed leukemia risk in children to the occupational exposure of their fathers to radiation.26

It appears that not all children have equivalent risk of developing cancer regardless of the mechanisms involved. For example, Canadian researchers are shedding light on the role of genetic variations in enzyme systems that metabolize carcinogens and subsequent susceptibility to childhood acute lymphocytic leukemia (ALL) from exposure to pesticides.27 Children with ALL frequently displayed alterations in a specific carcinogen-detoxifying gene. This genetic mutation was most common in children of mothers treated with a chemotherapy drug during pregnancy.28

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