The word chemotherapy means the use of any drug (such as aspirin or penicillin) to treat any disease, but to most people chemotherapy refers to drugs used for cancer treatment. It’s often shortened to “chemo.” Two other medical terms used to describe cancer chemotherapy are antineoplastic (meaning anti-cancer) therapy and cytotoxic (cell-killing) therapy.

 

Why chemotherapy is different from other treatments

Treatments like radiation and surgery are considered local treatments. They act only in one area of the body such as the breast, lung, or prostate and usually target the cancer directly. Chemotherapy differs from surgery or radiation in that it’s almost always used as a systemic treatment. This means the drugs travel throughout the body to reach cancer cells wherever they are.

Chemotherapy is used to treat many cancers. More than 100 chemotherapy drugs are used today — either alone or in combination with other drugs or treatments. As research continues, more drugs are expected to become available. These drugs vary widely in their chemical composition, how they are taken, their usefulness in treating specific forms of cancer, and their side effects.

 

How chemotherapy works

To understand how chemotherapy works, it helps to understand the normal life cycle of a cell, or the cell cycle. All living tissue is made up of cells. Cells grow and reproduce to replace cells lost through 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.

The cell cycle has 5 phases which are labeled below using letters and numbers. Since cell reproduction happens over and over, the cell cycle is shown 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 made from the first cell can go through this cell cycle again when new cells are needed.

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 from a few hours to a few years. When the cell gets a signal 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 ours.

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. The S phase lasts about 18 to 20 hours.

G2 phase: In the G2 phase, the cell checks the DNA and gets ready to start splitting into 2 cells. This phase 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.

The cell cycle is important because many chemotherapy drugs work only on cells that are actively reproducing (not cells that are 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.

Chemotherapy drugs cannot tell the difference between reproducing cells of normal tissues (those that are replacing worn-out normal cells) and cancer cells. This means normal cells are damaged and this results in 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).

 

The goals of chemotherapy

There are 3 possible goals for chemotherapy treatment:

 

Cure: If possible, chemotherapy is used to cure the cancer, meaning that the cancer disappears and does not return. However, most doctors do not use the word “cure” except as a possibility or intention. When giving treatment that has a chance of curing a person’s cancer, the doctor may describe it as treatment with curative intent. But there are no guarantees, and though cure may be the goal, it doesn’t always work out that way. It often takes many years to know if a person’s cancer is actually cured.

 

Control: If cure is not possible, the goal may be to control the disease — to shrink any cancerous tumors and/or stop the cancer from growing and spreading. This can help someone with cancer feel better and possibly live longer. In many cases, the cancer does not completely go away but is controlled and managed as a chronic disease, much like heart disease or diabetes. In other cases, the cancer may even seem to have gone away for a while, but it’s expected to come back.

 

Palliation: When the cancer is at an advanced stage, chemotherapy drugs may be used to relieve symptoms caused by the cancer. When the only goal of a certain treatment is to improve the quality of life but not treat the disease itself, it’s called palliative treatment or palliation.