What is radiation therapy?
When a person is diagnosed with cancer, three main methods of treatment may be used: surgery, radiation, and chemotherapy (drugs). Surgery and radiation are most useful if the cancer is a localized tumor whose shape and size can be determined. Radiation is the treatment of choice if the tumor is at an inoperable location, such as inside the brain, liver, or spine. In some cases, radiation is used following surgery in order to prevent recurrence of a malignant growth.
The amount of energy absorbed in radiation therapy can be expressed quantitatively in a unit called a gray. It was named after a British radiobiologist, Louis Harold Gray, who studied the biological effect of various ionizing radiations. One gray corresponds to the absorption of a fixed amount of radiation energy. Before 1984, radiation doses were commonly measured in rads, which was an acronym for radiation absorbed dose. The conversion factor is “100 rads = 1 gray.” Since most references continue to use the more familiar rad, it will be adopted here. For converting to grays, each rad value is divided by one hundred.
To develop a feel for typical dose levels in radiation therapy, it is helpful to remember two numbers for comparison: First, the maximum safe dose established by the Nuclear Regulatory Commission (NRC) for radiation exposure in the workplace is 5 rads per year; second, a lethal dose for 50 percent of humans, called LD-50, is about 450 rads. LD-50 means that, statistically, 50 percent would die and 50 percent would recover from such a massive dose.
The numbers given above must be interpreted with care. For example, a dental X-ray gives a dose of about 2 rads. Four dental X-rays give a total exposure of 8 rads, exceeding the NRC yearly maximum of 5 rads. Only a small part of the body, however, perhaps 1 percent, is irradiated during a dental X-ray. Therefore, the radiation dose averaged over the whole body is only 1 percent of 8 rads.
A patient with malignant cancer might be given twenty radiation treatments of 250 rads each, for a total dose of 5,000 rads. Even though this far exceeds the LD-50 of 450 rads, it is not lethal because it is spread out over time. The patient may develop symptoms of radiation sickness—skin reddening, nausea, loss of hair, and blood changes—but the body has time to heal partially between irradiations. The healing process can be compared to recovery from a serious burn injury, in which the skin and flesh repair themselves but permanent scars may be left behind. The present trend in radiology is to subdivide the total dose into more and smaller increments, reducing patient discomfort.
One of the most common elements used for cancer therapy is called cobalt 60, which is created by irradiating ordinary cobalt in a research reactor. Because the radioactive form of cobalt emits gamma rays with a penetrating power that is equivalent to a 2 million-volt X-ray machine, it is useful for the treatment of deep internal cancers. The radioactive source must be kept in a thick lead shield. When a patient is to be treated, the source is positioned near the cancer site, and a small port is opened briefly to irradiate it. Sometimes, the cobalt source is moved around the patient so that the gamma rays enter the body from different angles, thus reducing the damage to healthy tissue lying above the cancer site. The irradiation procedure is done by remote control so that the radiologist and other medical personnel will not be exposed.
Radioactive gold is another useful source for radiation therapy and one that is especially suitable for internal placement in the body, such as in the treatment of uterine or ovarian cancer. The half-life of radioactive gold is less than three days, so it can be left in the body to provide continuous treatment for a short time. The gamma rays from gold are of much lower energy than those from cobalt, so they penetrate only a few centimeters of tissue around the source. This limits damage to healthy cells farther from the irradiation site.
X-rays are seldom used today for cancer therapy because radioactive sources are much more convenient. Many radiopharmaceuticals, with various half-lives and energies, are available for different applications. Beams of electrons, protons, and other particles coming directly from nuclear accelerators also have been used to irradiate tumors, with some excellent results. The personnel and equipment costs to operate an accelerator, however, are too great for most hospitals. Also, patients may find the experience of being bombarded by the output beam from a large and noisy accelerator to be too traumatic.
The various types of radiation, including X-rays, gamma rays, and particle beams, all produce damage in living tissue by the process of ionization. The radiation strikes individual atoms and breaks the bonds that hold molecules together. The breakup of normal molecules produces positive and negative ions, which act as toxic chemicals. The internal structure of cells is disrupted so that they can no longer replicate themselves. Fortunately, cancer cells tend to be more sensitive to radiation damage than normal cells.
Radioactive needles, grains, and other sealed-source designs may be used when there is a suitable body cavity or opening. Such internal sources are common for treating cancer of the prostate, vagina, uterus, rectum, throat, and larynx. One problem with implanted sources is the radiation dose received by the physician during the surgery. Sometimes, it is possible to use “afterloading,” in which a hollow tube or shell is positioned in the patient and radioactive material is loaded into it at a later time.
Breast cancer is the second leading cause of death for American women. Early detection and treatment are the keys to improving the chances for survival. A radical mastectomy (breast removal) can be an emotionally traumatic experience. If the cancer is not too advanced, however, radiation therapy is an alternative. In some cases, daily treatment with a cobalt source can be done on an outpatient basis, with only minor disruptions of schedule and no long-term disfigurement.
During treatment for any type of cancer, the radiologist needs to determine whether the radiation therapy is shrinking the tumor. Therefore, computed tomography (CT) scanning or magnetic resonance imaging (MRI) will be used during and after treatment to monitor the size of the tumor. The radiation dose then can be adjusted to take into account the individual characteristics of various patients.
The goal of radiation therapy is to deliver a dose of several thousand rads to a cancer site while minimizing the damage to surrounding tissue and nearby organs. The shape of a malignant cancer may be a simple round lump, or it can be a complex group of nodules with tentacles extending through the flesh. For effective treatment, it is essential to determine the location and shape of the tumor as precisely as possible. An ordinary X-ray photograph is not very useful for diagnosing a tumor. The problem is a lack of contrast between a tumor and surrounding tissue, because there is very little difference in their densities. An X-ray of the head, for example, provides an excellent outline of the skull, but it cannot distinguish between tumor and brain material on the inside. In the 1970s, a major advance in X-ray technology called CT scanning was developed in England; it is sometimes also called a computed axial tomography (CAT) scan. CT scanning replaces the film of conventional X-rays with pictures on a computer screen.
A narrow X-ray beam with the diameter of a pencil scans across the region of interest, while an electronic detector measures the transmitted intensity and sends the data to the computer. A tumor is slightly denser than surrounding tissue, so a small decrease of intensity is recorded by the detector. The X-ray beam and detector system are rotated by a small angle, and another scan is recorded. Altogether, 180 scans may be used to record one “slice” of the body. The region where beams of decreased intensity from all the scans intersect defines the location of a tumor in that slice. Next, the X-ray beam and detector are moved along the body a small distance, and another 180 scans are made to obtain a second slice. Several more slices will be recorded. Eventually, the computer assembles all the information into a three-dimensional picture of the region of interest.
The sharpness and contrast obtained with CT scans are remarkable improvements over X-ray film. A radiologist can view the computer display of a tumor from various angles and with different enlargements. The computer screen can be photographed to provide a permanent record. The two scientists who developed the CT scan, Allan Cormack and Godfrey Hounsfield, shared the Nobel Prize in Physiology or Medicine in 1979.
An entirely different method to determine the location and outline of an internal tumor is called MRI. Instead of using X-rays, MRI utilizes the magnetic properties of hydrogen nuclei, which can be aligned by a very strong magnet.
The patient lies on a couch in a magnetic field produced by coils around his or her body. Radio waves are used to reverse the direction of alignment of the hydrogen. The concentration of hydrogen atoms in a tumor differs slightly from that in the surrounding tissue, so the output signal will differ correspondingly. Several methods are used to identify the precise location in the body where the magnetic reversals occur. A computer is used to convert the information into a pictorial display on a screen. Since bones contain virtually no hydrogen, the MRI picture shows them as shadows while emphasizing the structure of soft organs, tumors, and tissue.
Both MRI and CT scanning are complex diagnostic procedures. To diagnose and treat a cancer, radiologists, medical physicists, physicians, and computer specialists must work together as a team. New techniques for obtaining and displaying information are under continuing development.
The way in which radiation therapy is used depends on many factors, such as tumor location, the age and overall health of the patient, and the extent and stage of the cancer. Consider a young woman who has symptoms of back pain and loss of feeling in her legs. Use of MRI clearly shows a tumor growing inside her spinal column. For cancer in this location, treatment using radiation therapy, rather than surgery or chemotherapy, is indicated.
A treatment plan is devised that will minimize the radiation dose to the patient’s lungs, heart, and liver; the direction of entry for the beam of radiation must avoid passing through these sensitive organs. Furthermore, the radiologist has to make sure that the proper dose will be received at various depths below the skin. A plastic dummy to simulate the patient, called a phantom, can be irradiated with detectors placed inside it to measure the dose. Scattering of radiation and shielding effects by bones can be very complex and must be determined by computer calculations supplemented by careful measurements.
The radiation pattern can be shaped with filters and baffles so that it will conform to the shape of the tumor as closely as possible. A useful computer display for planning the therapy, called the “beam’s-eye view,” shows the tumor and its surroundings as if the observer’s eye were at the source of radiation.
Two final considerations in treatment planning for this patient are how the total radiation dose is to be subdivided and what the time interval between exposures should be. The radiologist must make a decision based on a judgment of the patient’s stamina, as well as of the urgency of treatment.
Consider another patient, a man who has developed a cancerous tumor of the tongue. Surgery is undesirable because his speaking ability would be impaired. Instead of an external radiation beam, the radiologist will probably recommend that radioactive needles be inserted directly into the affected region. Five to ten needles containing radioactive radium or cesium deliver the appropriate dose directly to the cancer site. The needles are left in place for several days and then removed. An alternative to needles in this case is the use of very small grains of radioactive material that can be implanted into the tumor. Sources with a short half-life, such as radioactive gold or radon, lose almost all their activity within two weeks, so the grains do not have to be removed.
The overall effectiveness of radiation therapy can be summarized approximately by the “half-half-half” rule. About half of all cancers are treated with radiation. Half of those patients are given a large enough dose to attempt a cure. (For the others, radiation is used simply for pain relief.) Finally, about half of these patients are actually cured by radiation. This success rate is encouraging, but clearly there is much room left for improvement.
The survival rate after radiation treatment varies greatly depending on the site of cancer in the body. For example, patients with localized cancer of the prostate, larynx, or uterus have five-year survival rates that range around 90 percent. At the other end of the scale, stomach or lung cancers that have spread and are at a much later stage show only about a 10 percent survival rate. Radiation therapy for ten different kinds of cancer—breast, cervix, larynx, prostate, uterus, bladder, testicle, tongue, mouth cancers, and Hodgkin disease—results in cure rates that are equal to or greater than those with surgery while preserving the organ function.
Many people are apprehensive about the hazards of radiation, with good reason. In the 1920s, some workers who were hired to apply radium paint to watch dials (to make them glow in the dark) developed cancer when they ingested radioactive material. At Hiroshima and Nagasaki, many people developed radiation sickness and died from the aftereffects of the atomic bombs dropped on those cities. For medical applications of radiation, regulatory agencies that are responsive to the general public must evaluate potential benefits and risks. Sometimes, sensationalized articles are published that present frightening scenarios of highly unlikely hazards. The perception of risk from low-level radioactive waste, for example, far exceeds the actual hazard. The advantages of radiation therapy would be lost if permits for hazardous waste storage were denied. The responsible use of any technology should balance concerns for safety with the benefits of that technology.
Ensuring the health and safety of medical personnel who administer radiation therapy to patients requires proper training. All workers must wear radiation monitors, which are checked daily. Radiation areas have to be posted with warning signs. The nuclear medicine department at a hospital must keep an accurate inventory of radioactive materials. The shipment and disposal of radiopharmaceuticals are strictly regulated. Periodic on-site visits by Nuclear Regulatory Commission inspectors also are part of the licensing procedure. Careless overexposure of personnel must be avoided if the benefits of radiation therapy are to find continued acceptance by the medical profession and the general public.
Radiation therapy most commonly makes use of X-rays or radioactivity. It is interesting to note that both types of radiation were discovered only a year apart in the 1890s. Wilhelm Röntgen, a German physicist, discovered X-rays in 1895. He received the Nobel Prize for his work in 1901, the first year in which the award was given. He used high voltage to accelerate an electron beam; when the electrons hit a metal target, they released a new kind of penetrating radiation. Röntgen made a now-famous X-ray photograph of his wife’s hand that clearly showed the bones inside the flesh. The medical profession adopted X-rays with great enthusiasm, primarily for the diagnosis of broken bones, swallowed objects, and bullet or shrapnel fragments.
Radioactivity was first observed in 1896 by Henri Becquerel, in Paris. By chance, Becquerel had placed a uranium rock next to unused photographic film that was still wrapped in its container. He was amazed to find that radiation from the uranium had penetrated the wrapping and had exposed the film inside. His discovery and follow-up experiments earned for him the Nobel Prize in 1905.
Marie Curie, a graduate student under Becquerel, became famous for isolating a new radioactive element from uranium ore, which she named radium. It emits radiation at a rate that is a million times greater than that of an equivalent weight of uranium. In her doctoral thesis in 1904, Curie described an experiment in which she placed a small capsule containing radium on her husband’s arm. It produced a sore that took more than a month to heal. The hazards of handling radioactivity and the possibility of using it to destroy cancer cells were recognized quite early.
The element radium is very rare on earth. In fact, the world’s total supply is less than 1 kilogram. Only after the invention of nuclear particle accelerators and neutron sources in the 1930s was it possible to create artificial radioactive elements in substantial amounts.
Radiation therapy has since become a treatment of choice in oncology. Such treatment will likely remain necessary for some time to come: It is not reasonable to expect a cure for cancer soon because there are too many different types. Much has been accomplished, however, in the area of prevention. The strong correlation between lung cancer and smoking has received wide publicity, so many people have stopped, at least in the United States. Research with animals has linked cancer to certain food additives and industrial pollutants, leading to legal restrictions on their use. In addition to surgery, radiation, and chemotherapy, other treatment methods are under investigation. For example, one procedure involves blocking the blood supply from reaching a tumor so that the malignant cells die from lack of nutrients. Other researchers hope to use genetic engineering, trying to stimulate the body’s immune system to produce specific antibodies that will fight against the cancer cells.
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