Radiation therapy for cancer
The use of ionizing electromagnetic energy particles or waves to destroy cancer cells. Radiation is the emission of energy in a pattern of rays, such as visible light. Ionizing radiation is a form of energy of sufficient intensity to alter the electronic charge of atoms and the structure of molecules, such as ultraviolet light. At high exposure, such alteration changes the structures of cells. The effects of ionizing radiation on cellular structure can cause as well as treat cancer.
The substances that contain the ionizing energy particles are radioactive isotopes, also called radionuclides or radioisotopes, most of which occur in the natural environment though scientists can cultivate them in the laboratory for consistency and ready availability. As the radioisotope disintegrates it releases radiation at a known rate, which allows the radiation oncologist to determine the appropriate exposure times and frequencies.
|RADIOISOTOPES USED IN RADIATION THERAPY|
|cesium-137 (137Cs)||cobalt-60 (60Co)|
|gold-198 (198Au)||iodine-125 (125I)|
|iodine-131 (131I)||iridium-192 (192Ir)|
|palladium-103 (103Pd)||phosphorus-32 (32P)|
|radium-226 (226Ra)||yttrium-90 (90Y)|
The radiation oncologist determines which radioisotope or type of radiation to use based on the type of cancer, its location in the body, and how extensively the tumor has spread. Some types of radiation are more effective for penetration to tumors deep within the body and others are more effective for treating tumors close beneath the surface of the SKIN. How rapidly the radioisotope dissipates is important for internal radiation therapy in which the radiation oncologist implants radioactive pellets into the body to directly expose the cancer tumor to the radiation.
How Radiation Therapy Works to Treat Cancer
A key characteristic of cancer cells is that they divide rapidly and without much organization. Radiation therapy works by damaging the DNA within the cells, which prevents them from dividing. Though all cells in the body are vulnerable to such damage, radiation therapy selectively targets the tumor, limiting the exposure of healthy cells to the radiation. Because normal cells do not divide as rapidly as cancer cells and they divide in an organized process, they are able to recover from the radiation exposure. The exceptions are the cells of fast-growing tissues such as skin and HAIR, which may experience some damage as a result of radiation therapy. The radiation oncologist structures shields and blocks to protect healthy tissue from the radiation as much as possible.
A fundamental premise of radiation therapy is fractionation-dividing a lethal DOSE of radiation into numerous sublethal doses administered over a period time. The standard protocol for radiation therapy in the United States delivers the fractionated dosage of radiation daily five days a week for two to nine weeks. Depending on the cancer and the capabilities of the treatment facility, some radition therapy protocols use radioisotopes and delivery methods that allow fewer doses administered over a shorter time and that vary the intensity of radiation according to the type and location of the cancer.
Types of Radiation Therapy
There are two basic types of radiation therapy, external beam and internal radiation. Some people receive both external beam and internal radiation therapy, depending on the type and location of their cancer. In external beam radiation therapy, the source of radiation is outside the body, directed toward the tumor using a machine. The radiation oncologist determines the precise point at which the radiation needs to enter the body, called the treatment portal, and places small tattoo dots to mark its boundaries. These tattoos are permanent and serve as the template for aligning the radiation delivery path.
The machine that delivers the radiation therapy is either a linear accelerator (which is most common) or a cobalt machine. These are similar in appearance to a large X-RAY machine. When receiving radiation therapy the person lies on a table beneath the machine, often positioned with supports and blocks to maintain the proper alignment for the radiation to hit the tumor. Each treatment session may take 15 to 30 minutes, though the actual delivery of radiation takes only a few minutes.
Radiation therapy does not hurt or cause any discomfort, though the experience can be somewhat stressful for people who are claustrophobic (become uncomfortable in closed spaces) because the machine is very large and often very close during treatment. Because the source of the radiation is outside the body, the person receives only the directed energy and does not become radioactive. External radiation therapy is often among the treatments for LUNG CANCER, BREAST CANCER, PROSTATE CANCER, COLORECTAL CANCER, Hodgkin’s lymphoma, THYROID CANCER, pharyngeal cancer, and some types of brain cancer.
INTRAOPERATIVE RADIATION THERAPY
Another form of external beam radiation therapy is intraoperative radiation, in which the person receives radiation to the surgical bed (site where the surgeon removed the tumor). Intraoperative radiation takes advantage of direct exposure to the site of the cancer to destroy any cancer cells that may have penetrated the tissue surrounding the tumor.
Internal radiation therapy, also called radiation seeding or brachytherapy, more directly targets the tumor with radioactive pellets (radioisotopes encased in thin wire containers) about the size of grains of rice, implanted in the body into or very near the tumor. Internal radiation therapy delivers a higher dose of radiation more directly to the tumor site, and often for a shorter duration, than would be possible with external beam radiation therapy. Internal radiation may be among the treatments for breast cancer, ENDOMETRIAL CANCER, thyroid cancer, CERVICAL CANCER, prostate cancer, and some cancers of the head and neck.
Internal radiation therapy may be
- interstitial, in which the radiation oncologist implants the radioactive pellets into the tumor or the tissue surrounding the tumor
- intracavitary, in which the radiation oncologist inserts the radioactive pellets into a natural body cavity such as the UTERUS or RECTUM
- intraluminal, in which the radiation oncologist inserts the radioactive pellets into a natural body passage such as the ESOPHAGUS or VAGINA
The implantation generally takes place with the person under general, regional, or local ANESTHESIA. After implantation the person is radioactive-that is, he or she emits ionizing radiation that can expose other people to its effects. Sometimes it is necessary to restrict contact with other people until the end of the course of treatment when the radioisotope dissipates enough to emit a level of ionizing radiation that is within safe limits. Sometimes the surgeon implants the pellets after an OPERATION to remove the tumor. An internal radiation implant remains in place for a few days to several weeks in most circumstances, though may remain for a few minutes to a few hours when the dose of radiation is very high and indefinitely when the optimal therapy is low-dose radiation over an extended time.
Risks, Side Effects, and Complications of Radiation Therapy
About half of people who have cancer receive radiation therapy during the course of their treatment. The general short-term side effects of radiation therapy include
- damage to the skin in the treatment area, similar to SUNBURN
- damage to hair follicles in the treatment area, resulting in local thinning or loss of hair
- mild NAUSEA
- tiredness and fatigue
Short-term side effects generally go away after the course of radiation therapy ends. Short-term risks, which are uncommon, include radiation BURNS to the skin and damage to tissues and organs in the treatment area that impairs their function. Long-term risks and complications of radiation therapy include destruction of the BONE MARROW, development of other types of cancer (notably LYMPHOMA and MULTIPLE MYELOMA), and permanent damage to tissues in the treatment area such as skin and MUSCLE. Specific types of radiation therapy have additional risks.