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

Questions Commonly Asked about Cancer in Children and Teenagers

ABOUT THE DIAGNOSIS

WHAT IS CANCER?

Cancer is a collection of diseases that have in common uncontrolled cell growth and the ability to invade the body. Cancer can spread locally (in the region where it arose) and travel to sites of the body distant from where it arose. Distant spread is called metastasis. The ability to spread makes cancer a malignant disease. Malignant means that the disease, if not treated, can progress and lead to death.

To begin to understand the complexities of why cancer appears, it is necessary to know something about cell growth. The body contains several types of tissue such as muscle, bone, bone marrow (where blood is made) and blood. Tissues are composed of cells. Normal cells respond to specific chemical messages from other cells that either stimulate or inhibit their growth.(135) Normal cells have a life span; they originate from primitive (young) cells called stem cells, reproduce (proliferate/divide), mature at varying rates and die. Mature cells cannot reproduce. Cells that die are generally replaced by new cells; this normal process is called renewal.

Most cancers originate in one or more young cells that have lost their built-in instructions to mature. Cancer cells reproduce repeatedly, replicating (copying) the abnormalities that have occurred. These cells are said to be immortal because, in contrast to normal cells, they appear to have an unlimited ability to reproduce. These altered cells do not respond to the normal chemical messages sent to them; they grow, even when their surroundings contain none of the signals normally required to stimulate growth. Their growth is out of control. As a result, they tend to reproduce at a rate which exceeds the rate needed to replace dying cells. They can form tumours (clusters of abnormally growing cells) and/or invade tissues, including bone marrow and blood.

Cancers are classified by both the tissue of origin and the cell type. For example, osteo in osteosarcoma means the cancer arose in bone. Sarcoma is a term used for tumour cells that resemble embryonic connective tissue cells. In general, a tumour that does not invade is considered benign (not cancer, not malignant). Some tumours are of borderline malignancy; under some conditions they will remain in a local area, but under other conditions - often poorly understood - they may acquire the capacity to invade and spread to other areas of the body. Some tumour cells continue to modify their properties as they grow.

HOW IS CANCER DIAGNOSED?

Cancer in children can be difficult to diagnose. In the majority of children with cancer, the early symptoms, such as fatigue, irritability, aches and fever, are non-specific and mimic those of common childhood illnesses. Cancer is suspected when such symptoms persist or progress and/or when certain findings are noted during a physical examination.

Blood tests and various types of X rays or scans are ordered to find the problem. However, the definitive diagnosis of cancer is usually made by microscopic and other sophisticated examinations of either a small piece of the tumour obtained by biopsy or a sample of bone marrow.

Once the diagnosis has been made, further tests determine whether the cancer has spread to other parts of the body. This is called staging. There are several methods for staging a cancer and each is specific to a particular type of cancer.(9,56) A detailed description of these methods is beyond the scope of this book. Staging information is important for planning treatment and predicting the likelihood of response to it; widespread cancer necessitates more intensive therapy.

However, the disease may be more advanced than the tests indicate. This is because more than a billion cancer cells must be present for the cancer to be detectable by the tests currently available.(127) If only a few cancer cells have spread to other parts of the body, they cannot be detected until they proliferate to large numbers. It is for this reason that most cancers in children are treated with chemotherapy, even in the early stages when the number of cancer cells is small.

HOW COMMON IS CANCER IN CHILDREN AND TEENAGERS?

Cancer in a person younger than 20 years is rare. Each year in Canada, it occurs in approximately 16 of every 100,000 children and teenagers. Yet, despite its relative rarity, cancer is second only to accidents as the most common cause of death in the age group five to 19 years.(117-124) It is projected that in 1996, approximately 1330 Canadian children and teenagers will be diagnosed with cancer and approximately 240 will die from it (Figure 1 (21K)).

The most common cancers in Canadian children and teenagers are leukemias (cancers of blood-forming cells), lymphomas (cancers of lymphatic tissues) and tumours of the central nervous system.

Combined, these account for approximately 60 percent of the cancers that occur in this age group. Other types of childhood cancer include bone and soft tissue sarcomas, neuroblastoma (a tumour of the sympathetic nervous system), Wilms tumour (a type of kidney cancer), germ cell tumours, retinoblastoma (a tumour of the eye), and hepatic (liver) cancers. Each of the more common cancers in children and teenagers is described briefly in a later section of this book.

Overall, cancer occurs slightly more frequently among boys than girls (Figure 2 (33K)). Kidney and epithelial cancers are exceptions in that they occur more frequently among girls than boys.

The occurrence of cancer in the young varies fourfold worldwide.(96) This variation results from the distribution of genetic and environmental factors that influence the development of cancer, as well as from differences in diagnostic, registration and health care practices. Canada is among the countries reporting the highest rates of childhood cancers. This may be explained partly by the completeness of cancer registration in Canada.

In children and teenagers, the incidence of all cancers combined is highest in the first five years of life and in the age group 15 years and older (Figure 3 (17K)). The greatest diversity of cancer types is seen in infants less than one year of age. In infants, nine types of cancer account for approximately 95 percent of all cancers, compared to seven types in children and teenagers aged 10 years and older (Figure 4 (32K)).

The variations by age in the frequencies of the different types of cancer produce age-specific incidence patterns (Figure 5 (19K)). Retinoblastoma and cancers of the sympathetic nervous system, kidney and liver occur most frequently in infants. The incidence of lymphoma, epithelial tumours and bone sarcoma increases with age. Leukemia and central nervous system tumour incidence is highest between the ages of one and four years.

IS CANCER SIMILAR IN YOUNG PEOPLE AND ADULTS?

Cancer is much less common in children than in adults (Figure 6 (29K)). Also, the types of cancer seen most often in children differ from those in adults. Epithelial cancers represent fewer than 10 percent of cancers in young people under the age of 20 years, compared to most of the cancers that occur in adults. These cancers arise in epithelial tissues covering the inner and outer body surfaces, including the glands. The most common epithelial cancers arise in the colon, lung and breast.

The causes of cancer in children and adults also appear to differ. Cancer-causing exposures to dietary, lifestyle and environmental factors appear to play a lesser role in cancers in children than in adults. Causes of cancer in children and teenagers are discussed in the next section of this book.

Many cancers in children are more successfully treated than cancers in adults. This may be due partly to the fact that children tolerate treatment better than adults. It also may be explained by the types of cells involved in cancers in children; the relatively faster growth of cancer cells in children makes them more susceptible to chemotherapy and radiation.

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