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External radiation therapy

External beam radiotherapy is the most common form of radiation treatment. It uses a machine called a linear accelerator to direct high-energy radiation beams to the exact location of the cancer.

Although each patient’s treatment course is different, most people receive treatments five days a week for 1 to 8 weeks, with breaks on the weekends. Your MRO radiation oncologist will determine 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 was first used in the 1950s and it revolutionized the way radiation was delivered to treat cancer. With the introduction of the multi-leaf collimator (MLC), we are now able use a linear accelerator to automatically sculpt radiation beams to conform to the shape of a tumor. The collimator design allows us to adjust the shape of the radiation, and move the linear accelerator to treat the tumor from several different angles, making it possible to deliver a prescribed dose across all three dimensions of the tumor.

3-D Radiation Therapy

In radiation therapy, our goal is to deliver radiation only to the cancer site, sparing as much normal, healthy tissue as possible. To do that, we use three-dimensional conformal radiation therapy (3-D CRT). We start by taking a CT scan during the initial simulation to produce a 3-D picture of the tumor, as well as nearby tissues. Then we use computer modeling to create a customized radiation plan, which enables us to conform the radiation beams to match the shape of the tumor. Using 3-D CRT, we aim many radiation beams at the tumor from different angles in order to spare as much normal tissue as possible. This results in a higher likelihood of a cure, and fewer potential side effects.


Intensity-modulated radiation therapy (IMRT) is an advanced form of three-dimensional radiation therapy. It allows our MRO care team to aim precisely shaped beams from the linear accelerator at the tumor from several directions, and adjust the intensity of the beams.

IMRT radiation may also be delivered with a continuous arc of radiation that is constantly being shaped to conform to the target and your anatomy. This is often called RapidArc or VMAT (Volumetric Arc Therapy). Another advantage of arc-based oncology is that your treatment time can be much shorter.

IGRT and precision localization techniques

Image-guided radiation therapy, or IGRT, involves daily imaging immediately before and/or during a treatment to verify the precise positioning of the target and radiation beams. Because tumors can move slightly from day to day, or even from moment to moment during a treatment (such as a lung tumor moving when you breathe), we use IGRT to make precise adjustments. This enables us to optimize your daily treatment, and minimize the dose to nearby normal tissues.

In addition to IGRT, MRO therapy centers are equipped with other techniques to ensure precise localization. For example, in certain cases, we can directly monitor the external contour of a patient’s body, to ensure precise positioning and account for motion during breathing. These techniques are important during breast cancer treatment, helping to avoid exposing the heart to radiation.