Intensity modulated radiation:
A type of external beam radiation therapy. As the name implies, the intensity of the radiation beam is modulated in some way. The reason we do this is to make sure that the radiation treatment is fully administered to the area of the body where the tumor is located, but also to reduce the radiation dose to the areas of the body that do not have cancer, which reduces the side effects of treatment. IMRT radiation therapy allows radiation dose escalation which has been shown to improve the cure rates and the effectiveness of treatment in general. The technique utilizes multiple beams or arcs which focus upon the target, thus the area of focus receives the full dose while the other tissues have much less radiation exposure. The characteristic of this treatment is the customization of radiation dose within the body.
There are 2 ways to deliver intensity modulated radiation therapy.
- Sliding Windows delivers treatment through multiple beams that are in fixed angles, but the intensity of the beam is modulated by the placement of a radiation attenuator in specific locations within the beam of radiation. This allows the full dose to travel through in specific paths but less than the full dose in other paths through the body. This technique has been utilized in various fashions for approximately 30 years.
- Dynamic Arc intensity modulated radiotherapy (DynArc) delivers radiation using arcs. An arc is the movement of the linear accelerator around the isocenter. Remember that a circle has 360 degrees, thus an arc can encompass any number of degrees but the important thing is that the radiation beam is turned on during the movement. The intensity of radiation is modulated during each arc by adjusting the rate of radiation delivery for each arc. This can be turned up if there are no sensitive structures between the tumor and the linear accelerator, or turned down if a critical structure comes between the linear accelerator and tumor. Thus, a customized dose of radiation is shaped by the variation in radiation dose between the different arcs. This technique has evolved over the last 20 years and has become commonly utilized since approximately 2011. Dr. Sharda has been routinely using this technique since 2003 and has the longest experience with dynamic arc therapy in the Las Vegas Valley.
IMRT requires specialized equipment and training, as well as meticulous attention to patient positioning as even a small variation can alter the dose to either the tumor or critical normal body tissues. IMRT requires complex treatment planning using high end computers and high-energy radiation machines. A CT scan is used to find and map the exact area to be treated. This helps ensure that the normal tissue is identifies and can be spared from radiation.
When treating areas with Intensity Modulated Radiation Therapy, multiple custom molds are used to ensure that the patient does not move during therapy. Here is an example of the custom molds used. No invasive fixation is necessary. The precision of treatment is to within 3-5mm.
Linear Accelerator delivery of IMRT is versatile, quick and accurate for the majority of body parts, with patient comfort and quick treatment time.