About Causes - This Battle Which I Must Fight: Cancer in Canada's Children and Teenagers

Questions Commonly Asked about Cancer in Children and Teenagers

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ABOUT CAUSES

HOW AND WHEN DOES CANCER START?

The development of cancer is a complex multi-step process. In many cases, its origins may lie in mutations (changes) in the chromosomes or genes of a cell.(135) Chromosomes are strands of genes arranged in linear order in the nuclei (centres) of cells. Genes carry information that determines the characteristics and actions of a cell. In particular, two classes of genes are critical in determining whether a cell is normal or malignant - those that promote cell growth and those that inhibit it.

Changes to chromosomes can result in missing or extra genes, genes in the wrong position on the chromosome, or genes that carry incorrect information. Some leukemias appear to result from a single genetic change; however, current evidence suggests that the development of most tumours involves progression towards increasing degrees of malignancy as the result of a number of mutations occurring one after the other.

The terminology used to describe the origins of cancer can be confusing. In this book, genetic refers to altered genes or chromosomes. Genetic changes can be either inherited or sporadic. Hereditary refers to changes in a parent's germ cells (sperm and eggs/ova) that can be passed on to the child. These changes are in every cell of the child's body, including his or her germ cells. In contrast, a sporadic genetic change is a chance event that occurs in an individual cell. The change then appears in all cells that are descendants of this original altered cell but not in other cells. This change is not passed on to the affected person's child unless the change occurred in a germ cell.

Familial means that several individuals in a family are affected. Familial occurrences of a disease probably result from heredity but may be aggravated by environmental factors to which the family is exposed. Finally, congenital means present at birth.

The time of origin for many childhood cancers is not yet known. The first few cells of Wilms tumour, neuroblastoma, germ cell and some brain tumours may be present at birth. Often these tumours are not detected until early childhood because the child usually feels well until the tumour is quite large, has spread to other areas, or causes symptoms. Even then, a tumour in an internal organ may be difficult to find by physical examination.

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WHAT CAUSES CANCER?

The chain of events that eventually leads to cancer is complex, and it is not yet possible to describe completely and accurately what causes a specific cancer; however, we are sometimes able to identify risk factors for specific cancers. A risk factor is a characteristic of an individual or the environment that is associated with a greater occurrence of disease than is expected by chance. By identifying risk factors, the possibility then exists for removing or modifying exposures and thereby reducing the occurrence of cancer. Epidemiology is the science that studies the distribution of disease to identify risk factors.

Many possible risk factors for the development of cancer in young people have been investigated, including physical, chemical and infectious agents, and genetic and immunologic factors; however, only a few risk factors for childhood cancer have been identified, and these account for a small proportion of cases. These factors can exert their effects at different periods in development: before conception (because of inherited susceptibility or the influence of various agents on the father's and mother's germ cells), during pregnancy (via the mother's exposures), and after birth.

Ionizing radiation is known to increase the risk of cancer.(18,92,93) Increased risks for many cancers have been seen in populations exposed to nuclear explosions or fallout. Low dose radiation therapy, used in the past to treat various conditions of the head, neck and chest, including tinea capitis (ringworm) and thymus enlargement, has been found to increase the risk of developing acute leukemias, brain tumours and cancers of the thyroid and parotid glands.(50) A mother's exposure to diagnostic X rays during pregnancy increases the risk of acute lymphoblastic leukemia in the child.(70,125) Fortunately, exposure to X rays is less now than in the past, since low dose radiation is used less frequently for conditions other than cancer, and improvements in equipment and shielding procedures protect patients from unnecessary exposure.

Over the past 15 years, some studies have examined the possible relationship between proximity to power lines or power-frequency electromagnetic fields and the occurrence of childhood cancers, particularly acute leukemias and brain tumours.(129,133,137) Electric and magnetic fields are everywhere and are generated from all components of our electric power system. They have an energy level many times less than ionizing radiation. People are exposed to these fields from many sources: transmission and distribution power lines, building wiring, electrically powered appliances and machinery. There is suggestive evidence of a statistical relationship between exposure to power lines and childhood leukemias and brain tumours; however, the quantity and quality of the available evidence is insufficient to conclude that power lines or magnetic fields cause cancer.(133) A Canadian study is underway to ascertain the significance of these factors.

Children can be exposed in several ways to chemical agents used in their neighbourhoods or in their parents' work environments. They can be affected by exposure of their parents' germ cells that occurred before they were conceived, during pregnancy by substances passed on to the fetus via the mother, or during childhood through contact with substances adults carry home from the workplace or use in homes and schools. However, the nature of parental occupational exposures that might be affecting cancer risk is only speculative at this time. For example, some studies have shown associations between brain cancer in children and their fathers' employment in hydrocarbon-associated occupations and the petroleum and chemical industries. Also, exposure of fathers to paint has been associated with both brain tumours and leukemia in their children. In contrast, other studies have not shown any such associations.(112)

Unusual exposures to infectious agents have been suggested as explanations for some clusters of childhood leukemias, and associations have been noted between compromised immunity and the occurrence of childhood lymphomas. However, the data are sparse and little is understood about the mechanisms of such associations. More research is needed in this area.

Retinoblastoma - a rare cancer of the eye - is the clearest example of an inherited tumour. Forty percent of children with retinoblastoma have the inherited form of the disease. The remaining 60 percent have the sporadic form that arises in the child and is not inherited. Some other childhood cancers are associated with specific birth defects or malformation syndromes (a group of defects seen together).(66,83,89) The likelihood that a childhood cancer is inherited is small in the absence of other genetic conditions.(34)

Research into the causes of childhood cancer is ongoing, but progress is slow because of the rarity of childhood cancer and difficulties in defining subgroups of cancers that likely have the same etiology (causes). As classification schemes evolve and more large-scale collaborative studies are undertaken, more may be learned about the factors involved in the development of cancers in childhood. The Canadian Childhood Cancer Surveillance and Control Program has been developed to better understand the causes of childhood cancer by undertaking carefully conducted epidemiologic studies.

Although not directly pertinent to cancer in children and teenagers, prevention of cancer in later life requires the development of healthy behaviours among young people. Children and teenagers should avoid exposures known to cause cancer in adults. In particular, these are smoking, excess exposure to sunlight and early sexual activity, particularly with multiple partners.(80) Healthy behaviours that should be encouraged include physical activity and a diet low in fat and high in fresh fruits and vegetables.

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IS CANCER A DISEASE OF MODERN TIMES?

The earliest descriptions of cancer in humans, including children, are in ancient Egyptian writings.(38) The cancers described were most often cancers of the breast, lips and tongue - cancers that could be seen or felt easily. The identification of cancers that are more difficult to characterize did not occur until much later. Leukemia, for example, was not recognized as a definite entity until 1845.(130)

Hippocrates (460-370 BC) first used the terms carcinos and carcinoma to describe cancer. The word carcinoma is derived from the Greek karkinos or crab; the Latinized form is cancer. Use of derivatives of the word crab to describe malignant tumours may have resulted from any of three observations: first, a well-advanced tumour is often fed by large veins which may resemble the claws of a crab. Secondly, the physician, Paul of Aegina (AD 629-690), noted that tumours are often difficult to remove because they adhere "like a crab". Thirdly, as some cancers spread they grow appendages that resemble the claws of a crab.

Relatively speaking, our understanding of how cancer is caused began only recently. It was not until the 1700s that the association between cancer and increasing age was documented and the observation made that different cancers appear to have different causes. In the Western World, the first accounts linking cancers to specific causes were made in 1762 (tobacco snuff) and in 1775 (soot).(130) One hundred years later the actual carcinogen in soot (benzopyrene) was identified.

The creation of the first hospital for cancer patients in Reims, France in 1740, the first use of radiation therapy as a treatment in 1896, the 1899 opening in Buffalo, New York, of the first laboratory devoted specifically to cancer research, and the introduction of chemotherapy after the Second World War all paved the way for cancer treatment as we know it today. (130)

The discovery of the first anti-leukemia agent, aminopterin, by Dr. Sidney Farber in Boston in the early 1950s led to the modern era of leukemia treatment.(142) In its relatively short history, chemotherapy has experienced its greatest successes in the young. In the 1970s, Dr. Don Pinkel of St. Jude Children's Research Hospital in Memphis showed that chemotherapy could induce longer remissions in leukemia and that cure was possible. This news provided the incentive for the formation of the international childhood cancer clinical trial groups. The work of these groups is largely responsible for the huge gains that have been made over the past two decades in treating cancer in the young.

 

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