External Beam Radiation Therapy
During external beam radiation therapy, a beam (or multiple beams) of radiation is directed through the skin to the cancer and the immediate surrounding area in order to destroy the main tumor and any nearby cancer cells. To minimize side effects, the treatments are typically given five days a week, Monday through Friday, for a number of weeks. This allows doctors to get enough radiation into the body to kill the cancer while giving healthy cells time to recover.
The radiation beam is usually generated by a machine called a linear accelerator. The linear accelerator, or linac, is capable of producing high-energy X-rays or electrons for the treatment of your cancer. Using treatment planning computers and software, your treatment team controls the size and shape of the beam, as well as how it is directed at your body, to effectively treat your tumor while sparing the surrounding normal tissue.
Several special types of external beam therapy are discussed this sections. These are used for specific types of cancer, and your radiation oncologist will recommend one of these treatments if he or she believes it will help you.
Three-Dimensional Conformal Radiation Therapy (3D-CRT)
Tumors are not regular; they come in different shapes and sizes. Three-dimensional conformal radiation therapy, or 3D-CRT, uses computers and special imaging techniques such as CT, MR orPET scans to show the size, shape and location of the tumor as well as surrounding organs. Your radiation oncologist can then precisely tailor the radiation beams to the size and shape of your tumor with multileaf collimators or custom fabricated field-shaping blocks. Because the radiation beams are very precisely directed, nearby normal tissue receives less radiation and is able to heal more quickly.
Intensity Modulated Radiation Therapy (IMRT)
Intensity modulated radiation therapy, or IMRT, is a specialized form of 3D-CRT that allows radiation to be more exactly shaped to fit the tumor. With IMRT, the radiation beam can be broken up into many “beamlets,” and the intensity of each beamlet can be adjusted individually. Using IMRT, it may be possible to further limit the amount of radiation received by healthy tissue near the tumor. In some situations, this may also safely allow a higher dose of radiation to be delivered to the tumor, potentially increasing the chance of a cure.
Proton Beam Therapy
Proton beam therapy is a form of external beam radiation treatment that uses protons rather than electron X-rays to treat certain types of cancer and other diseases. The physical characteristics of the proton therapy beam allow the radiation oncologist to more effectively reduce the radiation dose to nearby healthy tissue. Proton therapy is available at only a few centers in the country and is used in unique clinical situations.
Neutron Beam Therapy
Like proton therapy, neutron beam therapy is a specialized form of external beam radiation therapy. It is often used to treat certain tumors that are radioresistant, meaning they are very difficult to kill using conventional X-ray radiation therapy. Neutrons have a greater biologic impact on cells than other types of radiation. Used carefully, this added impact can be an advantage in certain situations. Neutron therapy is available at only a few centers in the country.
Image Guided Radiation Therapy (IGRT)
Radiation oncologists use image guided radiation therapy, or IGRT, to help better deliver the radiation to the cancer since tumors can move between treatments due to differences in organ filling or movements while breathing. IGRT involves conformal radiation treatment guided by imaging, such as CT, ultrasound or X-rays, taken in the treatment room just before the patient is given the radiation treatment on a daily basis.
All patients first undergo a CT scan as part of the planning process. The information from the CT scan is then transmitted to a computer in the treatment room to allow doctors to compare the earlier image with the images taken just before treatment. During IGRT, doctors compare these images to see if the treatment needs to be adjusted. This allows doctors to better target the cancer while avoiding nearby healthy tissue. In some cases, doctors will implant a tiny marker in or near the tumor to pinpoint it for IGRT. This helps to account for organ/tumor motion even if the body is immobilized by a casting device.