Radiation oncology is a medical field in which physicians, known as radiation oncologists, have been specially trained to treat cancer using radiation therapy, in which high doses of radiation are used to stop cancer-cell growth and division. They work as part of a treatment team that often includes general practitioners, surgeons and medical oncologists. Radiation therapy is a local treatment, meaning that only the cells in a specific area are targeted, and surrounding healthy tissue remains intact.
How Radiation Oncology Works
Radiation is delivered from an outside source during external-beam-radiation therapy, or from an implanted source during internal radiation therapy (brachytherapy). Cancer cells treated with radiation are unable to reproduce because their DNA is damaged. When they die, the cells are absorbed harmlessly by the body. With both external and internal radiation, the goal is to destroy the tumor, but protect healthy tissue from damage. The kind and duration of radiation therapy recommended depends on a number of factors, including the type and extent of the tumor being treated. Types of cancer that are frequently treated with radiation therapy include breast, brain, prostate, head and neck, colorectal and lung.
Types of Radiation Therapy
Although radiation therapy alone is an effective treatment for some types of cancers, others are best treated using radiation combined with surgery, chemotherapy and/or immunotherapy.
External-Beam-Radiation Therapy
During external-beam-radiation therapy (EBT), the patient is carefully positioned on a special couch, and the tumor is exposed to a beam of high-energy radiation produced by a machine called a "linear accelerator."
The first step in EBT is simulation, during which the patient is put in what will become the treatment position. X-rays or CT scans are performed to determine the tumor's exact size and location. Small marks may be made on the patient's skin. The radiation oncologist then works with a dosimetrist to decide how much radiation will be delivered, and for how long the beams will be left on during treatment. This is called treatment planning. In the final stage of EB (treatment delivery), the patient receives these precisely measured doses of radiation for an average of once a day, 5 days a week, for 2 to 9 weeks.
The length of time a patient is exposed to radiation during EBT is limited (usually only a few minutes), but treatment can take half an hour or more because the patient must lie in a specific position to ensure that radiation is focused on the tumor and not on healthy tissue.
Three-dimensional conformal radiotherapy
Tumors come in many different shapes and sizes. To help determine exactly where to train the beams of radiation during treatment, CT and MRI scans are performed to produce detailed three-dimensional images of the target area. This preparation process is used in three-dimensional conformal radiotherapy (3-D CRT) to minimize the risk of irradiating healthy tissue around the tumor.
Intensity-modulated-radiation therapy
Intensity-modulated-radiation therapy (IMRT) is a special kind of EBT wherein multiple beams of radiation with varying intensities are focused on all or part of a tumor. Special scans (3-D CT, MRI or PET) and computer-generated dose measurements are used to create a three-dimensional map of the tumor and determine the pattern and intensity of the radiation beams delivered during treatment. IMRT permits more precise targeting of cancerous tissue than traditional radiation therapy, and allows more radiation to be delivered to the tumor with fewer side effects or damage to surrounding healthy cells.
Stereotactic radiosurgery
Stereotactic radiosurgery is a kind of EBT used for tumors and lesions in the brain. Cancer cells are targeted using beams of radiation from special machines, whether gamma knife, linear accelerator or cyclotron. Accuracy is achieved with the use of three-dimensional computer images, and a helmet-shaped device that keeps the patient's head in position. Radiation is typically delivered in one session; however, the physician may recommend more than one session, in which case treatment is referred to as "stereotactic radiotherapy." Irradiated tumors shrink over the next few months or years.
Internal Radiation Therapy
Internal radiation therapy (brachytherapy) involves temporarily or permanently implanting a radioactive source to treat tumors from within the body. Radioactive sources may be placed near the tumor (intracavitary treatment) or in the tumor itself (interstitial treatment). Implantation is usually performed under general anesthesia, and requires a short hospital stay afterward.
External-beam-radiation therapy is painless. With internal radiation treatment, an anesthetic is given to the patient to prevent any pain during the implantation.
Side Effects of Radiation Therapy
Once radiation therapy has been completed, its side effects usually end within a few weeks. Hair loss is only an issue for patients who get radiation to the scalp or brain. Common side effects of external-beam-radiation therapy include dry, itchy, blistering and peeling skin, and fatigue. There are long-term side effects, including development of a second cancer, that can occur months or even years after treatment ends. The occurrence of a second cancer from radiation therapy is low, and any risk is usually outweighed by the benefit gained from treating the current cancer.