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Chemotherapy involves the use of drugs to destroy cancer cells. Although an ideal drug would destroy cancer cells without harming normal cells, most drugs are not that selective. Instead, drugs are designed to inflict greater damage on cancer cells than on normal cells, typically by using drugs that affect a cell's ability to grow. Uncontrolled and rapid growth is characteristic of cancer cells. However, because normal cells also need to grow, and some grow quite rapidly (such as those in the bone marrow and those lining the mouth and intestine), all chemotherapy drugs affect normal cells and cause side effects.
Although a single chemotherapy drug may be effective against some types of cancer, often doctors give several chemotherapy drugs at the same time (combination chemotherapy—see Combination Cancer Therapy).
One newer approach to limiting side effects and increasing effectiveness uses a variety of "molecularly targeted" drugs. These drugs kill cancer cells by attacking specific pathways and processes vital to the cancer cells’ survival and growth. For example, cancer cells need blood vessels to provide nutrients and oxygen. Some drugs can block blood vessel formation to cancer cells or the master signaling pathways that control cell growth. Imatinib, the first such drug, is highly effective in chronic myelocytic leukemia and certain cancers of the digestive tract. Erlotinib and gefitinib target receptors located on the surface of cells in non–small cell lung cancer. Molecularly targeted drugs have proven useful in treating many other cancers, including breast and kidney cancers.
Not all cancers respond to chemotherapy. The type of cancer determines which drugs are used, in what combination, and at what dose. Chemotherapy may be used as the sole treatment or combined with radiation therapy, surgery, or both.
In an attempt to increase the tumor-destroying effects of cancer drugs, the dose may be increased and the time between cycles of therapy may be decreased (dose-dense chemotherapy). Dose-dense chemotherapy, with shortened rest periods, is routinely used in breast cancer treatment. High-dose chemotherapy is often used for treatment of people whose cancer has recurred after standard dose chemotherapy, particularly for people with myeloma, lymphoma, and leukemia. However, high-dose chemotherapy can cause life-threatening injury to the bone marrow. Therefore, high-dose chemotherapy is commonly combined with strategies to protect the bone marrow (rescue). In bone marrow rescue, bone marrow cells are harvested before the chemotherapy and returned to the person after chemotherapy. In some cases, stem cells can be isolated from the bloodstream rather than from the bone marrow and can be infused back into the person after chemotherapy to restore bone marrow function.
Chemotherapy commonly causes nausea, vomiting, loss of appetite, weight loss, fatigue, and low blood cell counts that lead to anemia and increased risk of infections. People also often lose their hair, but other side effects vary according to the type of drug.
These symptoms can usually be prevented or relieved with drugs (antiemetics). Nausea may also be reduced by eating small meals and by avoiding foods that are high in fiber, that produce gas, or that are very hot or very cold. In some states, marijuana can be prescribed to relieve nausea and vomiting caused by chemotherapy.
Cytopenia, a deficiency of one or more types of blood cell, can develop because of the toxic effects chemotherapy drugs have on bone marrow (where blood cells are made). For example, a person may develop abnormally low numbers of red blood cells (anemia), white blood cells (neutropenia or leukopenia), or platelets (thrombocytopenia).
A person with anemia may be pale or have fatigue or weakness. People with more severe anemia may have dizziness, thirst, sweating, or even shortness of breath and chest pain. If anemia is severe, packed red blood cells can be transfused. A red blood cell growth factor, erythropoietin, also can be given, but transfusion is preferred because it has fewer side effects.
A person with neutropenia is at increased risk of developing an infection. A fever higher than 100.4° F in a person with neutropenia is treated as an emergency. Such a person must be evaluated for infection and may require antibiotics and even hospitalization. White blood cells are rarely transfused because, when transfused, they survive only a few hours and produce many side effects. Instead, certain substances (such as granulocyte-colony stimulating factor) can be administered to stimulate white blood cell production.
A person with thrombocytopenia is likely to bruise and bleed easily. If thrombocytopenia is severe, platelets can be transfused to lower the risk of bleeding.
Many people develop inflammation or even sores of the mucous membranes, such as the lining of the mouth. Mouth sores are painful and can make eating difficult. Various oral solutions (usually containing an antacid, an antihistamine, and a local anesthetic) can reduce the discomfort. On rare occasions, people need nutritional support by a feeding tube that is placed directly into the stomach or small intestine or even by vein.
Chemotherapy may cause loss of appetite. Corticosteroids may increase appetite and promote weight gain. Diarrhea may be a side effect of some chemotherapy drugs. Antidiarrheal drugs can be used as needed.
Sometimes chemotherapy drugs may damage other organs, such as the lungs, heart, or liver. For example, anthracyclines (such as doxorubicin), a type of topoisomerase inhibitor, cause heart damage when used in high total doses.
People treated with chemotherapy, particularly alkylating agents, may have an increased risk of developing leukemia several years after treatment. Some drugs, especially alkylating agents, cause infertility in some women and in most men who receive these treatments.
Some Chemotherapy Drugs
Tumor lysis syndrome may occur after chemotherapy because, when cancer cells are killed, they may release substances into the bloodstream. The syndrome occurs mainly in acute leukemias and non-Hodgkin lymphomas but can also occur after treatment of other types of cancer. The substances that circulate in the bloodstream may damage the kidneys or heart. Sometimes doctors are able to prevent tumor lysis syndrome by giving allopurinol before and during chemotherapy. Doctors may also give fluids by vein to cause the kidneys to excrete the substances quickly.
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