Chemotherapy is used to describe medications that treat cancer. In order to understand how these medications work, we need to understand a bit about tumor cells. Tumors are made up of cells that are reproducing at abnormally high rates. Normal cells know to stop reproducing (or dividing) when they come into contact with other cells. In the case of a tumor, this stop mechanism is missing, causing cells to continue to divide over and over. The RNA or DNA of a cell tell it how to replicate itself, and classic chemotherapy (which excludes immunotherpeutics and biological response modifiers) works by destroying this RNA or DNA. The more rapidly the tumor cells are replicating, the better chemotherapy is able to kill the cells.
Cell replication occurs in a series of phases, called the cell cycle. The cell cycle phases are: resting (G0; nothing is happening), G1 (or gap 1; a growth phase), S (synthesis; the replication of DNA occurs), G2 (gap 2; another growth phase), and M (mitosis; the actual division from 1 cell into 2). (See graphic of cell cycle)
Some chemotherapy agents are able to kill a cell during any phase of the cycle (these are called cell-cycle nonspecific), others are only able to kill during a specific phase and are unable to work in the resting phase (called cell-cycle specific). By giving cell-cycle specific agents at multiple time points, they are able to reach the maximum number of cells in the particular phase they affect. Therefore, these are most effective when given in divided doses (over multiple days or time points, for example: once a day for 5 days or every three hours for 4 doses) or by continuous infusion. Cell-cycle nonspecific drugs act against cancer cells at any phase of the cell cycle, including the resting phase. Cell-cycle nonspecific drugs are most effective when given in bolus doses (for example, over 20 minutes once). Cell death may not take place at the exact time the chemotherapy is given. Often a cell must undergo several divisions before it ultimately dies. Because not all the cancer cells die after a chemotherapy treatment, repeated doses are used to continue to reduce the number of cells.
Chemotherapy is considered a "systemic" therapy, meaning that it travels throughout the body, unlike surgery or radiation, which are "local" therapies. Doctors also use terms to describe when chemotherapy is given in the sequence of treatments:
Chemotherapy can be given in quite a few ways:
Doctors often combine specific and non-specific chemotherapies so that they complement each other and work together. This combination of different medications is called a "regimen". These regimens are often given names based on first letter of the medications used in them. For instance, CHOP, a common regimen for lymphoma, is made up of cytoxan (cyclophosphamide), adriamycin (hydroxydoxorubicin), oncovin (vincristine), and prednisone. This combination is given in "cycles" (blocks of time). For example, a cycle may be 21 days, and in the case of CHOP, cytoxan, adriamycin and oncovin are given on day 1, prednisone on days 1-5, followed by 16 days off (no treatment), for a total of 21 days, and then it starts over again with the next cycle.
There are different goals of chemotherapy treatment, depending on the site and stage of disease as well as other individual factors (other medical issues, previous treatments, functional status) that may impact the effectiveness of the treatment. Goals of chemotherapy include:
Cancer care teams use various methods to gauge if the treatment is working, depending on the type of cancer being treated. Many patients will have radiology studies (CT scans, MRIs, Pet scans) periodically to see if the tumor has responded (either shrunk or stayed the same versus gotten bigger or spread). Some types of tumors can be followed by checking a "tumor marker" in the blood. This is a substance that is either produced by the tumor or by the body in response to the tumor, and can be measured by a simple blood test. If the chemotherapy is working, one would expect the tumor marker to decrease. In some cases, a decrease in a patient's symptoms might be a sign of whether the medications are working against the tumor or not. Talk with your doctor or nurse about how your response will be measured.
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