Radiation therapy is one of the many treatments used to fight cancer. Radiation therapy may be used to try to cure a cancer or to relieve cancer symptoms by shrinking the tumor.
Radiation damages the DNA of cancer cells, and this damage kills the cells or prevents them from dividing to make new cancer cells. Unfortunately, radiation also damages nearby healthy cells, so several ways have been designed to deliver radiation to tumors while avoiding healthy tissue. Understanding the differences in these types of treatment can help patients and caregivers communicate with their doctors.
External-beam radiation therapy
External-beam radiation therapy (EBRT) delivers high-energy radiation from a linear accelerator machine outside the body directly to the tumor. The radiation beams can be made of photons (such as x-rays or gamma rays) or particles (such as electrons or protons). Delivering high-powered beams requires precision, and simulations based on computed tomography (CT), positron emission tomography (PET), or magnetic resonance imaging (MRI) scans provide doctors with the exact location and measurements of the tumor. EBRT sessions typically are short, but patients must remain still and are often fitted with body molds or casts to make sure the targeted body part moves as little as possible during the procedure. Radiation doses and treatments are tailored to the patient, type of cancer, tumor location, and tumor size. The full dose of radiation required to kill a tumor outright would cause too many negative side effects if delivered all at once, so the full dose is divided into smaller doses, called fractions. Patients typically receive fractions of EBRT 5 days per week for 4–6 weeks, until the full radiation dose is reached. There are many forms of EBRT, such as the following.
- Three-dimensional conformal radiation therapy (3D-CRT) uses multiple devices called collimators to shape radiation beams so that they resemble the shape of the tumor. The beams are delivered from multiple directions, and their shape helps reduce the damage to healthy tissue.
- Intensity-modulated radiation therapy (IMRT) is similar to 3DCRT, but in IMRT the collimators adjust the intensity of the beams as well as their shape. Adjusting the intensity of the beams reduces the radiation dose to nearby healthy tissue while increasing the dose to the tumor or specific parts of it.
- Image-guided radiation therapy uses repeated imaging scans (such as CT, MRI, or PET) during treatment to determine if the patient or radiation beams need to be repositioned during treatment in response to movement of the tumor. This technique is often used together with other types of EBRT.
- Proton therapy uses proton beams to kill cancer cells. Protons deliver the bulk of their energy at the end of their path, with little energy deposited in tissue along the way. Whereas photons may deliver a substantial radiation dose to the healthy tissue they pass through on the way to the tumor, protons deliver most of the radiation dose directly to the tumor.
- Stereotactic radiosurgery is often used to give a large dose of radiation to a small tumor area from multiple angles in a single session. This method is used only to treat small tumors with well-defined edges, such as those found in the brain and spinal cord.
Internal radiation therapy
Internal radiation therapy, or brachytherapy, uses radioactive sources sealed in “seeds” that are placed in the body through a needle or catheter. The radioactive seeds may be placed near a tumor or in a surgical cavity (the space left when a tumor is removed). In high-dose-rate brachytherapy, a highly radioactive source is placed in the body for a few minutes and then removed. In low-dose-rate brachytherapy, temporary or permanent radioactive seeds are placed in the body to continually give off radiation for days, weeks, or months as the radioactive source decays.
Systemic radiation therapy
In systemic radiation therapy, radioactive drugs called radiopharmaceuticals are given to patients through injection or by mouth. The radioactive substance sometimes is bound to a specially designed antibody (a monoclonal antibody) that seeks out and attaches to cancer cells, delivering radiation to the tumor site.
Radiation and other treatments
Radiation therapy may be used on its own or combined with other cancer treatments. Radiation is often given before surgery to shrink the tumor or given during or after surgery to kill off remaining cancer cells. Combining chemotherapy with radiation therapy (chemoradiation) is also now a standard treatment for many cancers. The drugs given often make the tumor more sensitive to radiation. Whether used alone or with other cancer treatments, the techniques described above have made radiation therapy safer and more effective than ever before.
– B. Strubberg
For more information, ask your physician, visit www.mdanderson.org or call askMDAnderson at 877-632-6789.
OncoLog, June 2016, Volume 61, Issue 6