Radiation uses high-energy radiation to shrink tumors and kill cancer cells. X-rays, gamma rays and charged particles are all types of radiation used for cancer treatment. Radiation therapy may be curative to certain types of cancer, if they are localized to one part of the body. It may also be used as part of adjuvant therapy to prevent tumor recurrence after surgery to remove a primary tumor. In some cases, radiation therapy is given with palliative intent, to relieve symptoms and reduce suffering cancer causes. Radiation therapy can be delivered by a machine outside the body, or come from radioactive material placed in the body near cancer cells. About half of all cancer patients receive some type of radiation therapy during the course of their treatment.
How does it work? Radiation therapy kills cancer cells by damaging their DNA, the molecules inside cells that carry genetic information from one generation to the next. This therapy either damages the DNA directly, or creates charged particles within the cells that then damage the DNA. When a cancer cell’s DNA is damaged beyond repair, the cell either stops dividing or dies. When damaged cells die, the body breaks them down and eliminates the cells through natural processes.
Because radiation therapy can also damage normal cells, doctors take potential healthy tissue damage into account when planning the amount of radiation needed. Doctors know the amount of radiation normal tissue in the body can safely receive, and use this information to decide the targeted area for treatment.
Types of Radiation There are various types of radiation therapy, and the type prescribed by a radiation oncologist depends on many factors. These factors include the type, size, and location of the cancer, the patient’s general health, how far the radiation must travel, how close the cancer is to normal tissue, and other factors such as the patient’s age and medical history. The following are all types of radiation therapy:
External-beam radiation therapy This radiation therapy is most often delivered in the form of photon beams. Patients usually receive external-beam radiation therapy in daily treatment sessions over the course of several weeks. Each patient’s number of treatment session varies, depending on specific factors. It is most often delivered using a linear accelerator machine (LINAC). The LINAC uses electricity to form a stream of fast-moving subatomic particles, which creates high-energy radiation.
- 3-Dimensional Conformal Radiation Therapy (3D-CRT) This external-beam radiation therapy uses advanced software and sophisticated treatment machines to deliver map the location of a tumor in 3 dimensions. The patient is fitted with a plastic mold or cast to keep the body part still during treatment, and radiation beams are matched to the shape of the tumor and delivered from several directions. This careful aiming of the beam may reduce radiation damage to normal tissue and increase the dose of radiation to the tumor.
- Intensity Modulated Radiation Therapy (IMRT) Like 3D-RT, this type of therapy locates the tumor in 3 dimensions. Along with aiming beams from different directions, IMRT can modify the intensity of those beams. This gives more control over the dose, decreasing the chance of radiation reaching normal tissue while delivering higher doses to the tumor. This therapy also uses a cast or mold to hold the body still during treatment. Because IMRT uses higher doses of radiation, it may increase the risk of second cancers developing later on.
- Tomotherapy Tomotherapy is a type of image guided IMRT. The machine used in tomotherapy is a mix between a CT imaging scanner and an external-beam radiation therapy machine. The part of the machine that delivers radiation for both imaging and treatment can rotate around the patient, much like a normal CT scanner. Tomotherapy machines capture CT images of the tumors immediately before sessions, to allow for extremely precise tumor targeting. Tomotherapy may be better than 3D-CRT at sparing normal tissue from high doses of radiation.
- Image-Guided Radiation Therapy (IGRT) During IGRT, repeated imaging scans are performed and processed by computers to identify changes in a tumor’s size and location. This allows adjustments of the position of the patient or planned radiation dose as needed. Repeated imaging can increase the accuracy of treatment, and can also decrease the total radiation dose to normal tissue.
- Intensity Modulated Proton Therapy (IMPT) IMPT is exactly like IMRT (Intensity Modulated Radiation Therapy), only using proton beams instead of photon beams. Protons can deliver radiation to the area that they are aimed, while doing less damage to nearby normal tissue. This therapy is often used for tumors near critical body structures such as the eye, brain and spine. Protons can only be sent out by a special machine, which costs millions of dollars and requires an expert staff. Because of this, proton therapy is expensive and offered only in few United States treatment centers.
- Stereoactic Radiosurgery (SRS) Stereoactic radiosurgery delivers one or more high doses of radiation to small tumors. SRS uses extremely accurate image-guided tumor targeting and patient positioning for optimal results. Because of this, a high dose of radiation can be given without excess damage to normal tissue. SRS requires the use of a head frame or other immobilization device to keep the patient still during therapy, which ensures that the high dose is delivered accurately. This type of therapy can be used to treat small tumors with well-defined edges, or more commonly, brain and spinal tumors.
Internal Radiation Therapy Internal radiation therapy is also known as brachytherapy, which means short-distance therapy. In this method of radiation, sources of radiation are put in or near the area that requires treatment. There is less risk of damaging normal tissue because this type of radiation only travels a short distance. Brachytherapy can deliver a high dose of radiation to a small area in a short amount of time, and is useful for tumors that are near tissue easily damaged by radiation, or that require higher doses.
There are two main types of internal radiation, interstitial and intracavitary. During interstitial radiation, the radiation source is placed directly into or next to the tumor using small pellets, seeds, wires, tubes or containers. In intracavitary radiation, a container of radioactive material is placed in a cavity in the body, such as the chest, rectum, uterus or vagina. X-rays, CT scans or ultrasounds are used to ensure the doctor places the radioactive source in the correct place.
There are two types of brachytherapy, permanent and temporary.
- Permanent Brachytherapy: Small containers about the size of a grain of rice, called pellets or seeds, are inserted into the tumors using thin, hollow needles. Once they are in place, the pellets give off radiation for weeks or months. Because they are so small, pellets are left in place after their radioactive material is used up.
- Temporary Brachytherapy: There are two types of temporary, high-dose rate (HDR) or low-dose rate (LDR). Both types place cylinders, hollow needles, catheters or fluid-filled balloons into the area requiring treatment. Radioactive materials are put into these containers for a short time, and then removed when finished. IN HDR, the radiation source is put into place for several minutes at a time, and then removed. In LDR, the radiation source stays in place for up to one week. LDR therapy requires staying fairly still, and patients receiving LDR therapy usually stay in the hospital during treatment.
Systemic Radiation Therapy In systemic radiation therapy, patients swallow or receive injections of radioactive substances, such as radioactive iodine. Radioactive iodine is commonly used to treat some forms of thyroid cancer because thyroid cells naturally take up the iodine. In other types of cancer, a radioactive substance bound to a monoclonal antibody helps target the radioactive substance to the right place. With the antibody joined to the radioactive substance, together they can travel through the blood to locate and kill tumor cells. Some systemic radiation therapy also drugs relieve pain from bone metastases. Many other systemic radiation therapy drugs are in clinical trials for different forms of cancer.
Side Effects Radiation can cause both early (acute) and late (chronic) side effects. Acute effects occur during treatment and chronic side effects can occur months or even years after treatment. Radiation side effects that develop can vary depending on the area of the body treated, the dose given, the patient’s medical condition and other treatments given at the same time. Whether or not a patient experiences late side effects depends on other cancer treatment in addition to their individual risk factors. Some chemotherapy drugs and lifestyle factors, such as smoking, can increase the risk of chronic side effects.
Acute radiation side effects are caused by damage to rapidly dividing normal cells in the area being treated. Skin irritation or damage can occur at regions exposed to radiation beams. For example, if a patient is being treated around the head or neck, damage to the salivary glands or hair loss can occur. The most common side effect of radiation therapy is fatigue, regardless of the area being treated. Nausea or vomiting is common when the abdomen or brain are undergoing treatment, but medications to treat nausea are available during radiation.
Chronic radiation side effects may or may not occur. Depending on the area of the body, late side effects can include damage to the bowels, memory loss, infertility and fibrosis. Sometimes, a second cancer can develop from radiation treatment.
When suggesting radiation therapy, the radiation oncologist first carefully weighs the known risks of treatment against the potential benefits for each patient. These benefits include relief of symptoms, shrinking a tumor, and potential cures. Hundreds of clinical trials, as well as personal experiences, help radiation oncologists decide which patients will benefit from radiation therapy. Discuss radiation therapy with your doctor to determine the most effective treatment for you.
The National Cancer Institute at the National Institutes of Health
The American Cancer Society