An Introduction to Image-Guided Radiation Therapy

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May 28, 2008 - 7:32 pm email to a friend printer friendly view / write comments

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Dr. Mehta

   Image Guided Radiotherapy, which his also known as IGRT, is a new and emerging technology in radiotherapy.

   In its broadest sense, IGRT applies to any of a number of technologies that improve the ability of the radiation oncologist to validate the patient’s exact position prior to initiating radiotherapy.  For many of years, the standard approach was to apply tattoos (little dots, not the interesting kind) to the patient’s body and then line the patient up on the treatment table by using wall-mounted lasers to verify the patient’s position.  For some treatments, a customized mold or cradle is created for the patient to lie in for each and every treatment.  This customized mold conforms to the patient’s shape and position and then solidifies.  In this way, the patient is thought to be in the same position for each and every treatment.  Periodically during the treatment, the patient undergoes a “port film”.  This is a simple x-ray that is taken during the course of radiotherapy.  The radiation oncologist evaluates this x-ray to ensure that the patient is accurately positioned on the treatment table.

   More recently, IGRT has come to mean the use of a CT scan performed periodically prior to the initiation of radiotherapy.  There are several different equipment platforms that perform this function.  The Elekta Synergy and the Varian Trilogy are linear accelerators that have a built in imaging device that resembles a CT scan.  The Tomotherapy unit is a CT scanner that has a built in linear accelerator.  Although there are subtle differences between the different platforms, the purpose of each is to image the patient’s soft tissues and more accurately evaluate the patient’s position prior to treatment.  By obtaining images that are nearly CT quality, there is a wealth more information than what can be seen on a simple x-ray image.  In addition, the CT that is obtained prior to treatment can be analyzed by a computer to compare how closely the patient is positioned on the table at the exact time of current treatment to the position of the patient on the table at the time of previous treatment planning and simulation.  The computer will also specify how to adjust the table in terms of height (up or down), right or left, and front to back in order to more accurately align the patient in three dimensions.  The accuracy of this CT matching is thought to be less than 3 mm.  The additional CT scans that are obtained during the treatment course add to the radiation exposure, but this is thought to be only a small amount of additional exposure.

   Another technique of aligning a patient prior to treatment includes the use of implanted fiducial markers, which are basically implanted landmarks.  The fiducial markers can either be gold beads, or metallic coils, or some other biologically inert item that can be implanted in or near the tumor.  The implantation of these types of markers can also help improve the accuracy of the setup.  These markers can show up on a simple x-ray.  By obtaining an x-ray or port film and comparing the location of the markers to where they were on the initial planning images, we can shift the patient on the table to a more accurate location.  Unfortunately, this approach does not allow the radiation oncologist to evaluate the soft tissues that are adjacent to the target tissue, but this marker-based imaging that is often described as 2D imaging can be performed quickly.

   The entire rationale for more accurate setup is fairly intuitive.  If the patient’s position can be effectively immobilized and/or the patient can be accurately positioned prior to treatment, the treatment will presumably be delivered precisely to the location intended.  The safety margin around the target that is included can be reduced if the inaccuracy or variation in daily setup can be reduced. 

   Improving accuracy in treatment setup has several advantages.  Firstly, it assures that the area intended to be treated is in fact receiving radiotherapy.  Secondly, if the accuracy of treatment can be assured, than the amount of the safety margin attributed to inaccuracies in daily setup can be reduced.  This reduction in the safety margin will reduce the normal tissues exposed to radiotherapy.  Because the radiotherapy is always administered to a volume of tissue, it is important to recognize that even a small reduction in the safety margin can result in a large sparing of the normal tissues.  By reducing the radius of the safety margin from 2 cm to 1 cm, the volume of normal tissue exposed to radiotherapy is reduced by 8 fold.

   IGRT utilizing CT imaging has another potential, but for the most part unrealized, advantage.  The CT imaging also provides a glimpse as to what is happening during the course of radiotherapy.  In some instances, the tumor is actually shrinking during the course of radiotherapy.  There are studies ongoing to determine whether it is safe to actually shrink the radiotherapy fields in response to a shrinking tumor.  Because of concern that there might be microscopic residual disease in the periphery separated by areas of dead tumor (meaning that the tumor mass doesn’t shrink perfectly symmetric in every direction), physicians are reluctant to shrink the fields until more data is accumulated.

   More later.  The field of IGRT is rapidly growing, and we’re only learning more about how to best use this technique.

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