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leukemia
Leukemia, also spelled leukaemia, is a group of cancers that usually begin in the bone marrow and result in high numbers of abnormal white blood cells.〔(【引用サイトリンク】url=http://www.cancer.gov/cancertopics/types/leukemia )〕 These white blood cells are not fully developed and are called ''blasts'' or ''leukemia cells''. Symptoms may include bleeding and bruising problems, feeling tired, fever, and an increased risk of infections.〔 These symptoms occur due to a lack of normal blood cells.〔 Diagnosis is typically made by blood tests or bone marrow biopsy.〔
The exact cause of leukemia is unknown. Different kinds of leukemia are believed to have different causes. Both inherited and environmental (non-inherited) factors are believed to be involved. Risk factors include smoking, ionizing radiation, some chemicals (such as benzene), prior chemotherapy, and Down syndrome.〔〔 People with a family history of leukemia are also at higher risk.〔 There are four main types of leukemia — acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), chronic lymphocytic leukemia (CLL) and chronic myeloid leukemia (CML) — as well as a number of less common types.〔(【引用サイトリンク】url=http://www.cancer.gov/researchandfunding/snapshots/leukemia )〕〔 Leukemias and lymphomas both belong to a broader group of tumors that affect the blood, bone marrow, and lymphoid system, known as tumors of the hematopoietic and lymphoid tissues.
Treatment may involve some combination of chemotherapy, radiation therapy, targeted therapy, and bone marrow transplant, in addition to supportive care and palliative care as needed. Certain types of leukemia may be managed with watchful waiting.〔 The success of treatment depends on the type of leukemia and the age of the person. Outcomes have improved in the developed world.〔 The average five-year survival rate is 57% in the United States.〔(【引用サイトリンク】 SEER Stat Fact Sheets: Leukemia )〕 In children under 15, the five-year survival rate is greater than 60 to 85%, depending on the type of leukemia.〔 In children with acute leukemia who are cancer-free after five years, the cancer is unlikely to return.〔(【引用サイトリンク】title=Survival rates for childhood leukemia )〕
In 2012, leukemia developed in 352,000 people globally and caused 265,000 deaths. It is the most common type of cancer in children, with three quarters of leukemia cases in children being the acute lymphoblastic type.〔 However, about 90% of all leukemias are diagnosed in adults, with AML and CLL being most common in adults.〔 It occurs more commonly in the developed world.〔
==Classification==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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