External Radiation Therapy
External beam radiotherapy is the most commonly used form of radiation treatment, in which a machine called a linear accelerator directs high-energy radiation beams to the exact location of the cancer.
Although each patient’s treatment course is different, most receive treatments five days per week for one to eight weeks, with breaks on the weekends. The radiation oncologist, a specially trained physician, will decide the dosage, technique, and type of radiation to be used for your treatment. Patients who receive external beam radiation therapy are not radioactive during or after treatment.
Multi- Leaf Collimators
The medical linear accelerator, first used in the 1950s, revolutionized the way radiation was delivered to treat cancer. However, until the more recent ability of digital diagnostic imaging, powerful computer work stations, and specialized software tools, it was virtually impossible to sculpt the radiation beam to the shape of the tumor.
The introduction of the multi-leaf collimator (MLC) has significantly contributed to recent advances in radiation therapy. The MLC is a specialized, computer-controlled device with as many as 120 tungsten fingers, or leaves, inside the linear accelerator. This device enables radiation beams to be shaped automatically to conform to the shape of the tumor. By automatically adjusting the shape and moving the linear accelerator to treat the tumor from several different angles, it is possible to deliver a prescribed dose across all three dimensions of the tumor.
3-D Treatment Planning
In radiation therapy, the goal is to deliver radiation only to the cancer site, sparing as much normal, healthy tissue as possible. Through evolving technologies, the treatment team is able to deliver a higher radiation dose to the tumor while avoiding or giving a lower dose to the nearby normal tissues. This results in a higher likelihood of a cure, and the lower dose to the normal tissues results in fewer potential side effects from the radiation treatment.
In three-dimensional conformal radiation therapy (3-D CRT), the initial simulation CT scan is used to produce a 3-D picture of the target, as well as nearby tissues of the body. Computer modeling is used to create a customized radiation plan, such that the beams are conformed to match the target shape. Many radiation beams may be aimed at the tumor from different angles in order to spare normal tissue as much as possible.
Intensity-Modulated Radiation Therapy (IMRT) is an advanced form of three-dimensional radiation therapy. Not only are precisely-shaped beams from the linear accelerator aimed at the target from several directions, but the intensity or strength of portions of the beams can be adjusted. This allows the physician to deliver higher concentrated doses of radiation to the target while substantially reducing exposure to normal tissues and organs, thereby decreasing the risk of side effects and complications.
IMRT is accomplished with specialized modern computers and optimization techniques. The radiation oncologist designs the dose distribution that is desired, and the computer determines the optimal way to deliver the treatment. This is termed inverse planning.
IMRT may also be delivered with a continuous arc of radiation that is constantly being shaped to conform to the target and your particular anatomy. This is often termed RapidArc or VMAT (Volumetric Arc Therapy). Another advantage of arc-based techniques is that treatment times can be much faster.
IGRT And Other Precision Localization Techniques
Image-guided radiation therapy, or IGRT, involves daily imaging immediately prior to treatment and/or during the treatment itself to verify the precise positioning of the patient, target, and radiation beams. This allows for optimal daily treatment, minimizing the dose to nearby normal tissues.
Furthermore, tumors can move slightly from day to day, or even from moment to moment during a treatment (such as a lung tumor moving with breathing motion). IGRT accounts for this, and adjustments are made on demand to precisely localize your treatment.
These techniques are based on the initial simulation CT scan that all patients undergo as part of the treatment planning process. The information from this scan is transmitted to a computer in the treatment room to allow doctors to compare the initial images with images taken at the time of daily treatment. In some cases, doctors will implant a tiny marker in or near the tumor to pinpoint it for IGRT. With these techniques, doctors are able to better target the cancer while avoiding nearby healthy tissue.
In addition to IGRT, our centers are equipped with other techniques to ensure precise patient localization. For example, in certain cases, we can directly monitor the external contour of a patient’s body, thereby ensuring precise positioning and accounting for breathing motion. These techniques are important, for example, during breath-holding techniques for breast cancer treatment in order to avoid exposing the heart to radiation.
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