Childhood cancer is cancer in a child. In the United States, an arbitrarily adopted standard of the ages used are 0–14 years inclusive, that is, up to 14 years 11.9 months of age. However, the definition of childhood cancer sometimes includes young adults between 15-19 years old. Pediatric oncology is the branch of medicine concerned with the diagnosis and treatment of cancer in children.
Incidence and mortality 
In the United States, cancer is the second most common cause of death among children between the ages of 1 and 14 years, exceeded only by accidents. More than 16 out of every 100,000 children and teens in the U.S. were diagnosed with cancer, and nearly 3 of every 100,000 died from the disease. In the United States in 2012, it was estimated that there was an incidence of 12,000 new cases, and 1,300 deaths, from cancer among children 0 to 14 years of age.
In developed countries, childhood cancer has a mortality of approximately 20% of cases. In low resource settings, on the other hand, mortality is approximately 80%, or even 90% in the world's poorest countries.
Internationally, the greatest variation in childhood cancer incidence occurs when comparing high-income countries to low-income ones. This may result from differences in being able to diagnose cancer, differences in risk among different ethnic or racial population subgroups, as well as differences in risk factors. An example of differing risk factors is in cases of pediatric Burkitt lymphoma, a form of non-Hodgkin lymphoma that sickens 6 to 7 children out of every 100,000 annually in parts of sub-Saharan Africa, where it is associated with a history of infection by both Epstein-Barr virus and malaria. In industrialized countries, Burkitt lymphoma is not associated with these infectious conditions.
Risk factors 
Familial and genetic factors are identified in 5-15% of childhood cancer cases. In <5-10% of cases, there are known environmental exposures and exogenous factors, such as prenatal exposure to tobacco, X-rays, or certain medications. For the remaining 75-90% of cases, however, the individual causes remain unknown. In most cases, as in carcinogenesis in general, the cancers are assumed to involve multiple risk factors and variables.
Aspects that make the risk factors of childhood cancer different from those seen in adult cancers include:
- Different, and sometimes unique, exposures to environmental hazards. Children must often rely on adults to protect them from toxic environmental agents.
- Immature physiological systems to clear or metabolize environmental substances
- The growth and development of children in phases known as "developmental windows" result in certain "critical windows of vulnerability".
The most common cancers in children are (childhood) leukemia (34%), brain tumors (23%), and lymphomas (12%). In 2005, 4.1 of every 100,000 young people under 20 years of age in the U.S. were diagnosed with leukemia, and 0.8 per 100,000 died from it. The number of new cases was highest among the 1–4 age group, but the number of deaths was highest among the 10–14 age group.
In 2005, 2.9 of every 100,000 people 0–19 years of age were found to have cancer of the brain or central nervous system, and 0.7 per 100,000 died from it. These cancers were found most often in children between 1 and 4 years of age, but the most deaths occurred among those aged 5–9. The main subtypes of brain and central nervous system tumors in children are: astrocytoma, brain stem glioma, craniopharyngioma, desmoplastic infantile ganglioglioma, ependymoma, high-grade glioma, medulloblastoma and atypical teratoid rhabdoid tumor.
Other, less common childhood cancer types are:
- Neuroblastoma (7%)
- Wilms tumor (5%)
- Non-Hodgkin lymphoma (4%)
- Rhabdomyosarcoma (3%)
- Retinoblastoma (3%)
- Osteosarcoma (3%)
- Ewing sarcoma (1%)
- Germ cell tumors
- Pleuropulmonary blastoma
- Hepatoblastoma and hepatocellular carcinoma
Organizations focused on childhood cancer through cancer research and/or support programs include: Childhood Cancer Canada, the Children's Cancer and Leukaemia Group (in the United Kingdom), Child Cancer Foundation (in New Zealand) and American Childhood Cancer Organization (in the United states).
Learning problems 
Children with cancer are at risk for developing various cognitive or learning problems. These difficulties may be related to brain injury stemming from the cancer itself, such as a brain tumor or central nervous system metastasis or from side effects of cancer treatments such as chemotherapy and radiation therapy. Studies have shown that chemo and radiation therapies may damage brain white matter and disrupt brain activity.
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