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Cancer Chemotherapy
Normally, your cells grow and die in a controlled way. Cancer cells keep forming without control. Chemotherapy is drug therapy that can stop these cells from multiplying. However, it can also harm healthy cells, which causes side effects.
During chemotherapy you may have no side effects or just a few. The kinds of side effects you have depend on the type and dose of chemotherapy you get. Side effects vary, but common ones are nausea, vomiting, tiredness, pain and hair loss. Healthy cells usually recover after chemotherapy, so most side effects gradually go away.
Your course of therapy will depend on the cancer type, the chemotherapy drugs used, the treatment goal and how your body responds. You may get treatment every day, every week or every month. You may have breaks between treatments so that your body has a chance to build new healthy cells. You might take the drugs by mouth, in a shot or intravenously.
What Is Chemotherapy?
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Although the word chemotherapy can mean the use of any drug (such as aspirin or penicillin) to treat any disease, to most people the term chemotherapy refers to drugs used for cancer treatment. Two other HeartCare-Info terms often used to describe cancer chemotherapy are antineoplastic (meaning anti-cancer) therapy and cytotoxic (cell-killing) therapy.
History of chemotherapy
The first drug used for cancer chemotherapy did not start out as a medicine. 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 group of people were accidentally exposed to mustard gas and were later found to have very low white blood cell counts. Doctors reasoned that an agent that damaged the rapidly growing white blood cells might have a similar effect on cancer. Therefore, in the 1940s, several patients with advanced lymphomas (cancers of certain white blood cells) were given the drug by vein, rather than by breathing the irritating gas. Their improvement, although temporary, was remarkable. That experience led researchers to look for other substances that might have similar effects against cancer. As a result, many other drugs have been developed.
Why chemotherapy is different from other treatments
Chemotherapy is often 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 medicines travel throughout the body to reach cancer cells wherever they may have spread. Treatments like radiation and surgery act in a specific area such as the breast, lung, or colon, and so are considered local treatments.
More than 100 drugs are used today for chemotherapy -- either alone or in combination with other drugs or treatments. As research continues, more drugs are expected to become available. These medicines vary widely in their chemical composition, how they are taken, their usefulness in treating specific forms of cancer, and their side effects. New medicines are first developed through research in test tubes and animals. Then, the drugs are tested in clinical trials in humans to find out how safe they are and how well they work.
How Does Chemotherapy Work?
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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. Cells grow and reproduce to replace cells lost during injury or normal "wear and tear." The cell cycle is a series of steps that both normal cells and cancer cells go through in order to form new cells.
This discussion is somewhat technical, but it can help you understand how doctors predict which drugs are likely to work well together and how doctors decide how often doses of each drug should be given.
There are 5 phases in the cell cycle, which are labeled below using letters and numbers. Since cell reproduction happens over and over, the cell cycle is shown below as a circle. All the steps lead back to the resting phase (G0), which is the starting point.
After a cell reproduces, the 2 new cells are identical. Each of the 2 cells that is made from the first cell can go through this cell cycle again when new cells are needed.
The Cell Cycle
- G0 phase (resting stage): The cell has not yet started to divide. Cells spend much of their lives in this phase. Depending on the type of cell, G0 can last for a few hours to a few years. When the cell is signaled to reproduce, it moves into the G1 phase.
- G1 phase: During this phase, the cell starts making more proteins and growing larger, so the new cells will be of normal size. This phase lasts about 18 to 30 hours.
- S phase: In the S phase, the chromosomes containing the genetic code (DNA) are copied so that both of the new cells formed will have matching strands of DNA. This phase lasts about 18 to 20 hours.
- G2 phase: In the G2 phase, the cell checks the DNA and prepares to start splitting into 2 cells. It lasts from 2 to 10 hours.
- M phase (mitosis): In this phase, which lasts only 30 to 60 minutes, the cell actually splits into 2 new cells.
This cell cycle is important to cancer doctors (oncologists) because many chemotherapy drugs work only on cells that are actively reproducing (not on cells in the resting phase, G0). Some 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. Doctors can also plan how often doses of each drug should be given based on the timing of the cell phases.
When chemotherapy drugs attack reproducing cells, they cannot tell the difference between reproducing cells of normal tissues (that are replacing worn-out normal cells) and cancer cells. The damage to normal cells can cause side effects. Each time chemotherapy is given, it involves trying to find a balance between destroying the cancer cells (in order to cure or control the disease) and sparing the normal cells (to lessen unwanted side effects). |
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