Chemotherapy is usually given by mouth (in pill or liquid form) or intravenously (directly into the vein). Administered in these ways the medications enter the bloodstream, which carries them to nearly all parts of the body that might contain cancer cells. Under special circumstances it might be advantageous to aim the medications more directly; this can be done by administering them directly into the artery that feeds a specific part of the body, into abnormal fluid accumulations in the chest or abdominal cavities, or into the spinal fluid that bathes the brain and spinal cord.
Anticancer drugs can be separated into classes according to their origin, chemical nature, and method of action. The most commonly used drug classes include the alkylating agents, antimetabolites, antitumor antibiotics, plant alkaloids, and hormones.
A major side effect of the first three drug classes is “bone marrow suppression,” and is usually the limiting factor in the frequency and dosage with which these drugs can be given safely. The bone marrow is contained in the center of bones (similar to the marrow in animal bones used to make soups) and produces the white blood cells that fight infections, the red blood cells (corpuscles) that carry life-sustaining oxygen to the body’s organs, and the platelets, which help the blood to clot. When the bone marrow is suppressed, fewer blood cells are produced and the patient becomes more vulnerable to infections, anemic and at risk for serious bleeding.
Before chemotherapy is given, a blood test is done to determine how many white blood cells, red blood cells, and platelets are circulating in the blood. The dosage of chemotherapy is calculated on the basis of the results of this blood count. After chemotherapy is administered, it is expected that the blood count will decrease over days to weeks (depending on the drugs used), reflecting the bone marrow’s suppression. After the marrow has recovered, the blood count will return to normal and the next course of chemotherapy can be given.