Radiation is a form of intense energy generated by a radioactive substance, such as cobalt, or by specialized equipment, such as an atomic particle (linear) accelerator.
Radiation preferentially kills cells that divide rapidly and cells that have difficulty repairing their DNA (nuclear material). Cancer cells divide more often than normal cells and often cannot repair damage done to them by radiation. Therefore, cancer cells are more likely than most normal cells to be killed by radiation. Nonetheless, cancer cells differ in how easily they are killed by radiation. Some cells are very resistant and cannot be effectively treated with radiation.
Types of Radiation Therapy
In its most common form, radiation therapy uses an external beam of gamma radiation generated by a linear accelerator. Less commonly, electron or proton beam radiation is used. Proton beam radiation, which can be focused on a very specific area, effectively treats certain cancers in areas in which damage to normal tissue is a particular concern, such as the eyes, brain, or spinal cord. All types of external beam radiation are focused on the particular area or organ of the body that contains the cancer. To avoid overexposing normal tissue, several beam paths are used and surrounding tissues are shielded as much as possible.
There are several newer methods of external beam radiation therapy. These techniques allow doctors to deliver higher doses of radiation and more precisely target cancer cells while shielding healthy cells from radiation. Some of these newer technologies include intensity modulated radiation therapy (IMRT), three-dimension conformal radiation therapy (3D-CRT), stereotactic radiosurgery, stereotactic radiotherapy, image-guided radiation therapy, and dynamic multileaf collimator therapy. These techniques involve the use of imaging (computed tomography, magnetic resonance imaging, and ultrasonography) to allow doctors to precisely locate tumors and shape radiation beams to the tumors. The choice of technique often depends on tumor location.
External beam radiation therapy is given as a series of equally divided doses over a prolonged period of time. This method increases the lethal effects of the radiation on cancer cells while decreasing the toxic effects on normal cells. Toxic effects are decreased because normal cells can repair themselves quickly between doses while cancer cells cannot. Typically, a person receives daily doses of radiation over a period of 6 to 8 weeks. To ensure that the same area is treated each time, the person is precisely positioned using foam casts or other devices.
In other radiation therapy strategies, a radioactive substance may be injected into a vein to travel to the cancer (for example, radioactive iodine, which is used in treatment of thyroid cancer). Another technique, called brachytherapy, uses small pellets ("seeds") of radioactive material placed directly into the cancer (for example, radioactive palladium used for prostate cancer). These implants provide intense radiation to the cancer, but little radiation reaches surrounding tissues. Implants contain short-lived radioactive substances that stop producing radiation after a period of time.
More recently, radioactive substances have been attached to proteins called monoclonal antibodies, which seek out cancer cells and attach to them. The radioactive material attached to the antibody concentrates at the cancer cells and destroys them.
Radiation therapy plays a key role in curing many cancers, including Hodgkin lymphoma, early-stage non-Hodgkin lymphoma, squamous cell cancer of the head and neck, seminoma (a testicular cancer), prostate cancer, early-stage breast cancer, some forms of non–small cell lung cancer, and medulloblastoma (a brain or spinal cord tumor). For early-stage cancers of the windpipe (larynx) and prostate, the rate of cure is essentially the same with radiation therapy as with surgery. Sometimes, radiation therapy is combined with other forms of treatment. Certain kinds of chemotherapy drugs, such as cisplatin, enhance the effectiveness of radiation therapy, and these drugs may be given with radiation treatments.
Radiation therapy can reduce symptoms when a cure is not possible, as for bone metastases in multiple myeloma and painful tumors in people with advanced lung, esophageal, head and neck, and stomach cancers. By temporarily shrinking the tumors, radiation therapy can relieve symptoms caused by spread of cancer to bone or brain.
Unfortunately, radiation can damage normal tissues near the tumor. Side effects depend on how large an area is being treated, what dose is given, and how close the tumor is to sensitive tissues. Sensitive tissues are those in which cells normally divide rapidly, such as skin, bone marrow, hair follicles, and the lining of the mouth, esophagus, and intestine. Radiation can also damage the ovaries or testes. Doctors try to accurately target the radiation therapy to prevent excessive damage to normal cells.
Symptoms depend on the area receiving radiation and may include fatigue, mouth sores, skin problems (such as redness, itching, and peeling), painful swallowing, lung inflammation (pneumonitis), hepatitis, gastrointestinal problems (such as nausea, loss of appetite, vomiting, and diarrhea), urinary problems (such as increased frequency and burning during urination). People may also have a low blood cell counts, leading to anemia (which causes fatigue and weakness), easy bruising or bleeding, and risk of infections. Radiation to head and neck tumors often causes damage to the overlying skin as well as the lining of the mouth and throat. Doctors try to identify and treat such symptoms as early as possible so the person remains comfortable and can continue with treatments. For example, a variety of drugs can reduce the diarrhea caused by radiation therapy to the abdomen.
Radiation therapy can increase the risk of developing other cancers years after the initial cancer was treated. The risk depends on the person's age at the time of treatment and the part of the body that received the radiation.