|* Leukemias and cancers of the brain and central nervous system account for more than half of childhood cancers (see Question 1).
|* The causes of childhood cancers are largely unknown (see Question 4).
1. What are the most common types of childhood cancer?
Among the 12 major types of childhood cancer, leukemias (blood cell cancers) and cancers of the brain and central nervous system account for more than half of the new cases. About one-third of childhood cancers are leukemias. The most common type of leukemia in children is acute lymphoblastic leukemia. The most common solid tumors are brain tumors (e.g. gliomas and medulloblastomas), with other solid tumors (e.g. neuroblastomas, Wilm tumors, and sarcomas such as rhabdomyosarcoma and osteosarcoma) being less common.
2. How many children are diagnosed with cancer in the United States annually?
In the United States in 2007, approximately 10,400 children under age 15 were diagnosed with cancer and about 1,545 children will die from the disease. Although this makes cancer the leading cause of death by disease among U.S. children 1 to 14 years of age, cancer is still relatively rare in this age group. On average, 1 to 2 children develop the disease each year for every 10,000 children in the United States.
3. How have childhood cancer incidence and survival rates changed over the years?
Over the past 20 years, there has been some increase in the incidence of children diagnosed with all forms of invasive cancer, from 11.5 cases per 100,000 children in 1975 to 14.8 per 100,000 children in 2004. During this same time, however, death rates declined dramatically and 5-year survival rates increased for most childhood cancers. For example, the 5-year survival rates for all childhood cancers combined increased from 58.1 percent in 1975-77 to 79.6 percent in 1996-2003. This improvement in survival rates is due to significant advances in treatment, resulting in a cure or long-term remission for a substantial proportion of children with cancer.
Long-term trends in incidence for leukemias and brain tumors, the most common childhood cancers, show patterns that are somewhat different from the others. Incidence of childhood leukemias appeared to rise in the early 1980s, with rates increasing from 3.3 cases per 100,000 in 1975 to 4.6 cases per 100,000 in 1985. Rates in the succeeding years have shown no consistent upward or downward trend and have ranged from 3.7 to 4.9 cases per 100,000.
For childhood brain tumors, the overall incidence rose from 1975 through 2004, from 2.3 to 3.2 cases per 100,000, with the greatest increase occuring from 1983 through 1986. An article in the September 2, 1998, issue of the Journal of the National Cancer Institute suggests that the rise in incidence from 1983 through 1986 may not have represented a true increase in the number of cases but may have reflected new forms of imaging equipment (magnetic resonance imaging or MRI) that enabled visualization of brain tumors that could not be easily visualized with older equipment. Other important developments during this time period included the changing classification of brain tumors, which resulted in tumors previously designated as "benign" being reclassified as "malignant," and improvements in neurosurgical techniques for biopsying brain tumors. Regardless of the explanation for the increase in incidence that occurred from 1983 to 1986, childhood brain tumor incidence has been essentially stable since the mid 1980s.
4. What are the known or suspected causes of childhood cancer?
The causes of childhood cancers are largely unknown. A few conditions, such as Down syndrome, other specific chromosomal and genetic abnormalities, and ionizing radiation exposures, explain a small percentage of cases.
Environmental causes of childhood cancer have long been suspected by many scientists but have been difficult to pin down, partly because cancer in children is rare and because it is difficult to identify past exposure levels in children, particularly during potentially important periods such as pregnancy or even prior to conception. In addition, each of the distinctive types of childhood cancers develops differently - with a potentially wide variety of causes and a unique clinical course in terms of age, race, gender, and many other factors.
A number of studies are examining suspected or possible risk factors for childhood cancers, including early-life exposures to infectious agents; parental, fetal, or childhood exposures to environmental toxins such as pesticides, solvents, or other household chemicals; parental occupational exposures to radiation or chemicals; parental medical conditions during pregnancy or before conception; maternal diet during pregnancy; early postnatal feeding patterns and diet; and maternal reproductive history. Researchers are also studying the risks associated with maternal exposures to oral contraceptives, fertility drugs, and other medications; familial and genetic susceptibility; and risk associated with exposure to the human immunodeficiency virus (HIV).
5. What have studies shown about the possible causes of childhood cancer?
For several decades, the NCI [National Cancer Institute], a part of the National Institutes of Health (NIH) has supported national and international collaborations devoted to studying the causes of cancer in children. Key findings from this research include the following:
- High levels of ionizing radiation from accidents or fromn radiotherapy have been linked with increased risk of some childhood cancers.
- Children with cancer treated with chemotherapy and/or radiation therapy may be at increased risk for developing a second primary cancer. For example, certain types of chemotherapy, including alkylating agents or topoisomerase II inhibitors (e.g. epipodophyllotoxins), can cause an increased risk of leukemia.
- Recent research has shown that children with AIDS (acquired immunodeficiency syndrome), like adults with AIDS, have an increased risk of developing certain cancers, predominantly non-Hodgkin lymphoma and Kaposi sarcoma. These children also have an additional risk of developing leiomyosarcoma (a type of muscle cancer).
- Certain genetic syndromes (e.g. Li-Fraumeni syndrome, neurofibromatosis, and Gorlin syndrome) have been linked to an increased risk of specific childhood cancers.
- Children with Down syndrome have an increased risk of developing leukemia.
- Low levels of radiation exposure from indoor radon have not been significantly associated with childhood leukemias.
- Ultrasound use during pregnancy has not been linked with childhood cancer in numerous large studies.
- Residential magnetic field exposure from power lines has not been significantly associated with childhood leukemias.
- Pesticides have been suspected to be involved in the development of certain forms of childhood cancer based on interview data. However, interview results have been inconsistent and have not yet been validated by physical evidence of pesticides in the child's body or environment.
- No consistent findings have been observed linking specific occupational exposures of parents to the development of childhood cancers.
- Several studies have found no link between maternal cigarette smoking before pregnancy and childhood cancers, but increased risks have been related to the father's smoking habits in studies in the United Kingdom and China.
- Little evidence has been found to link specific viruses or other infectious agents to the development of most types of childhood cancers, though investigators worldwide are exploring the role of exposures of very young children to some common infectious agents that may protect children from, or put them at risk for, developing certain leukemias.