How Chemotherapy Works

Chemotherapy is the use of drugs to treat cancer. Before surgery, chemotherapy may be used both to reduce the size of the breast tumor and to destroy cancer cells wherever they may be. After surgery, chemotherapy works throughout your system to kill cancer cells that may have spread throughout your body. Here's how this systemic treatment works.

Your body's normal cells grow and divide in a controlled manner. Cancer cells, however, grow and divide in total chaos—without any control or logical order. Chemotherapy works by stopping the growth or multiplication of cancer cells, thereby killing them. You may worry that chemotherapy will kill normal cells, too, and there is that possibility. However, remember that these drugs work best on cells that divide rapidly—namely, cancer cells. This makes chemotherapy particularly effective against cancer.

When used as systemic therapy right after surgery, chemotherapy has another advantage: being in the right place at the right time. Let's say that cancer cells have broken away from the primary tumor, and that these relatively young and small clusters are now located somewhere in your body. These single cells or small clusters have plenty of nutrients and oxygen, and they are dividing quite rapidly (by contrast, with larger tumors the cells are crowded together, there isn't enough food to go around, and the cells don't have the energy to grow). This is perfect timing for chemotherapy because, remember, chemotherapy works best on rapidly dividing cells. And this is why cancer cells are much more sensitive to chemotherapy than normal cells.

A "cycle" of chemotherapy refers to one time or one "round" in which you go to the doctor and receive the medication. A "course" of chemotherapy refers to all the cycles in your entire treatment. Depending on the drug (or drugs) you receive, you may have anywhere from four to eight cycles of chemotherapy during an entire course.