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CHEMOTHERAPY PRINCIPLES

작성자 닥터코리아 조회수 1786
CHEMOTHERAPY PRINCIPLES





WHAT IS CHEMOTHERAPY?





The thought of chemotherapy frightens many people. Understanding what to expect before, during, and after chemotherapy can calm many of these fears. This essay will help explain what chemotherapy is, how it works, and what to expect, in general, with chemotherapy.



To doctors, nurses, pharmacists, and health professionals, the word chemotherapy means any drug (like aspirin or penicillin) used for treating people with any disease. Cancer chemotherapy is understood to mean medications or drugs that destroy cancer cells. Most lay people, however, think of anti-cancer drugs when they hear the term "chemotherapy." Two of the medical terms often used to describe cancer chemotherapy are antineoplastic (anti-cancer) and cytotoxic (cell-killing).



The first drug used for cancer chemotherapy was not originally intended for that purpose. Mustard gas was used as a chemical warfare agent during World War I and was studied further during World War II. During a military operation in World War II, a large number of military personnel was accidentally exposed to that agent and were later found to have abnormally low white blood counts. It was reasoned that an agent that damaged the rapidly growing white blood cells might have a similar effect on cancer. Therefore, in the 1940's several patients with advanced lymphomas were given the drug (by vein, rather than by breathing the irritating gas). Their improvement, although temporary, was remarkable. That experience provided the impetus for the study of other substances that might have similar effects against cancer - as a result, many additional drugs have been developed to treat many other types of cancer.



Chemotherapy is sometimes the first choice for treating many cancers. It differs from surgery or radiation in that it is almost always used as a systemic treatment. This means the drugs travel throughout the whole body or system rather than being localized to one area such as the breast, lung, or colon. This is important because chemotherapy can reach cancer cells that may have spread to other parts of the body.



There are now well over 90 drugs used for chemotherapy and many more are expected to become available soon. These chemotherapy drugs vary widely in their chemical composition, how they are taken, their usefulness in treating specific forms of cancer, and their side effects. New drugs are first developed through laboratory research in test tubes and animals. Then, their safety and effectiveness are tested in three phases of clinical trials in humans.



Clinical trials are studies of new or experimental drugs (or other new treatments). The study is done when there is a reason to believe a new drug or a new combination of drugs may be of value in curing or controlling cancer. The study will be fully explained to the individual and their family and the individual has the opportunity to volunteer or refuse to participate in the study. Taking part in a clinical trial does not prevent anyone from receiving other medical or nursing care that is needed. Patients who participate in clinical trials make an important contribution to medical care because the study results will also help future patients. And, the participant will be the first to benefit from these new treatments. To learn more about clinical trials, you can contact the American Cancer Society (1-800-ACS-2345 or www.cancer.org) or the National Cancer Institute (1-800-4-CANCER or cancertrials.nci.nih.gov). The National Cancer Institute can provide a listing of clinical trials for people with various types and stages of cancer.



HOW CHEMOTHERAPY WORKS



To understand how chemotherapy works as a treatment it is helpful to understand the normal life cycle of a cell in the body. All living tissue is composed of cells and sustained by cell growth and reproduction that replaces cells lost during injury or normal "wear and tear." The cell cycle is a series of steps through which both normal cells and abnormal cancer cells grow and reproduce to form two new cells. There are 5 steps, called cell cycle phases, that are designated by letters and numbers:



G0, G1, S, G2, and M.



G0, G1, S, G2, and M.





The Cell Cycle

GO = Resting stage

G1 = RNA and protein synthesis

S = DNA synthesis

G2 = Construction of mitotic apparatus

M = Mitosis



Cells in the G0 phase, which is sometimes referred to as the resting phase, have not yet started reproducing. The length of the G0 phase varies from hours to years for different types of cells. When the cell is stimulated to reproduce (for instance, when other cells wear out and need to be replaced), it moves into the G1 phase. During this step, the cell's production of RNA and proteins increase. This phase lasts about 18-30 hours, and is followed by the S phase. During the S phase, the cell replicates its DNA so that both of the new cells it will eventually form will have the right amount of DNA. This phase lasts about 18-20 hours. G2, the period right before the cell starts the process of splitting into two cells, lasts from 2-10 hours. It is also a period when RNA and proteins are formed. The last step is called M for mitosis. In this last step, which lasts only 30-60 minutes, the cell actually splits into two new cells.



This cell cycle is important to oncologists because many cytotoxic chemotherapy drugs work only on actively reproducing cells (not G0, the resting phase). These drugs specifically attack cells in a particular phase of the cell cycle (the M or S phases, for example). Understanding how these drugs work helps oncologists predict which drugs are likely to work well together, and plan how often doses of each drug should be given.



Although chemotherapy drugs attack reproducing cells, they do not distinguish between reproducing cells of normal tissues (that are replacing worn-out normal cells) and cancer cells. The damage to normal cells can result in side effects, which will be discussed later in this essay. Thus, each time chemotherapy is given it involves a balance between destroying the cancer cells (in order to cure or control the disease) and sparing the normal cells (to minimize undesirable side effects).



GOALS OF TREATMENT WITH CHEMOTHERAPY



There are three main goals of chemotherapy treatment. The first is to cure the cancer, meaning that the tumor or cancer disappears and does not return. If this is not possible, the second goal is to control the disease (stop the cancer from growing and spreading) in order to provide the best quality of life for the person with cancer. Sometimes cure and control are not possible if the cancer is in an advanced stage. At this point the goal is called palliation. This means that chemotherapy drugs will be used to relieve symptoms caused by the cancer, thereby improving the patient’s quality of life, even though the length of their life may not be prolonged.



For some patients, chemotherapy is the only treatment used in an attempt to cure, control, or palliate their cancer. In other cases, chemotherapy may be given as neoadjuvant therapy before surgery or radiation, or as adjuvant therapy after surgery or radiation. Adjuvant chemotherapy is given to prevent the reproduction of stray cancer cells remaining in the body after surgery or radiation. In most cases, these stray cancer cells cannot be recognized by routine tests such as x-rays, but are assumed to be present. Neoadjuvant chemotherapy may be used to shrink a large tumor so that it can be removed by surgery or a less extensive operation can be used to accomplish that goal.