Many years of cancer research by scientists from many disciplines have led to certain principles which guide the chemotherapist in tailoring treatment of a specific cancer in a specific patient.
- Chemotherapeutic drugs do not distinguish between normal cells and cancer cells, both of which go through the cell cycle. Therefore a balance must be found between tumor destruction and the side effects caused by harming normal tissues.
- Most chemotherapy drugs act best against rapidly dividing cells that spend little or no time in the resting G0-phase of the cell cycle. This accounts for many of the side effects of these drugs, such as the loss of hair, bone marrow suppression, and the development of sores in the mouth and along the intestinal tract because hair, bone marrow cells, and the cells lining the mouth and other parts of the intestinal tract all divide rapidly.
- As tumor masses grow larger, more cells enter the resting G0-phase; the rate of division (and vulnerability to chemotherapy) therefore decreases. Thus chemotherapy is most effective against small tumors.
- Most of the chemotherapy now being used works by a mechanism known as “first-order cell kill.” This means that a given dose of drug will kill a given percentage (rather than a given number) of cancer cells. Take, for example, a drug with a kill rate of 90 percent and a tumor mass of one billion cells (a little smaller than your little fingernail). One course (administration) of the drug will kill 90 percent of the cancer cells, leaving 100 million cells. The second course will kill another 90 percent, leaving 10 million cells. Each successive course of chemotherapy will leave behind a viable 10 percent. Since even one remaining cell can multiply until the tumor mass reaches the critical size, cure of the cancer requires that therapy continue until that last cancer cell is destroyed.
- The greater the number of cancer cells, the more likely that some of those cells will become “resistant” (immune) to a given type of chemotherapy drug.
Using the general principles above, chemotherapists frequently employ several different classes of drugs in high doses. This is done in order to attack cells in different parts of the cell cycle, to circumvent cancer cell resistance to one class of drug, and to balance the drugs’ different toxicities. Enough time is scheduled between treatments to allow adequate recovery of normal cells, but not full recovery and renewed growth of the cancer cells.