X rays are similar in nature to light rays, but they travel at vastly higher energy levels and are invisible to the human eye. The cosmic rays that are detectable on a Geiger counter are natural forms of X rays from space and make up part of the background radiation to which all of us are exposed without identifiable ill effects. An ordinary chest X ray exposes the patient to about 120 times the usual background radiation, but this is still considerably less than the typical dosage given to treat a cancer patient.
In anticancer radiation therapy, the ultimate target for the radiation is the deoxyribonucleic acid (DNA) in the malignant cell. DNA is the genetic material controlling a cell’s reproduction. The DNA of a malignant cell is more susceptible to radiation damage than that of a normal cell. The radiation disrupts the structure of several of the chemical elements in the DNA portion of the cell nucleus, leading to cell death. This destruction of the cancer cell is not immediate; instead, it occurs over a period of hours or even days. Sometimes several exposures of the rapidly dividing cancer cells to the high-energy radiation are required, and even then enough tumor cells may survive to permit regrowth of the cancer.
To prevent this, the radiation therapist calculates the total amount of radiation that will probably be required to destroy the cancer, and then spreads that amount over a number of treatments. This increases the probability of killing all the tumor cells without excessive damage to normal tissues. In this way the cancer will be destroyed over a period of time, by killing the individual cancer cells at a faster rate than they can repair themselves and reproduce. To minimize further the radiation damage to the skin and other normal tissue, the radiation dosage will be directed at the tumor through several different “ports” or sites on the skin surface. In other words, the target is always the cancer, but the radiation may be beamed to it via different pathways.
In theory, no cancer exists that cannot be destroyed by radiation. The problem is delivering the required amount of radiation to the cancer without causing excessive harm to the patient. For example, if a radiation therapist knows that 6,500 rads will be required to destroy a certain type of cancer, a regimen course of treatment must be devised that will permit the delivery of this dosage to the cancer itself with only minimal exposure to the rest of the body. A single dose of only 700 rads administered to the entire body would prove fatal to more than half of the people receiving such exposure. Thus the task is to deliver the required amount of radiation to the cancer itself while sparing the healthy part of the body. This is not always possible for a variety of reasons.
As a result, part of the tumor may be destroyed, but enough cells will remain that can repair their X-ray injuries and then reproduce to form new tumors. Or some of the cancer may escape exposure to the radiation a situation referred to as a “geographic miss.” This sometimes occurs when there are multiple sites of cancer or when some cells on the outer reaches are not exposed to the full dose and are able to reproduce and spread.
Still another important factor in the outcome of radiation therapy is the nearness of the tumor to sensitive normal tissues or organs that cannot withstand exposure to high doses of radiation. A tumor along the spinal cord, for example, may be very difficult or impossible to treat with radiation because the X-ray dosage needed to destroy the tumor might also destroy the spinal cord. A weakened general physical condition caused by the cancer or other diseases such as diabetes, alcoholism, or kidney failure also may reduce a patient’s tolerance to radiation therapy. But even with these potentials for failure, large numbers of cancer patients can be effectively treated and even cured with radiation therapy.