Image-Guided Radiation Therapy (IGRT)

One requirement for the precise application of high radiation dosages to pre-determined target areas is being able to reliably position the patient in exactly the same way for each session. Spatial irradiation errors can result in a reduction of the dosage within the target volume as well as an increase in the dosage to critical healthy tissue such as the rectum and bladder in the case of a prostate treatment. Moreover, position variability necessitates the addition of safety margins to the irradiation target area. These safety margins include normal healthy tissue that is then also exposed to the prescribed (high) radiation dosage. In addition to the possibility of mis-positioning the patient, which can be minimized by appropriate fixation techniques, the variability in the position of the prostate and seminal vesicles within the pelvis - dependent on bladder and rectum content - must also be taken into account. Numerous methods to reduce position variability have been developed. In contrast to the more or less invasive methods such as the daily insertion of specially sized rectal balloons or the permanent implantation of gold fiducial markers which can be detected by x-ray, non-invasive methods are becoming increasingly available. Two of the non-invasive methods are offered by our department.
The computertomographic method and the ultrasound based method. Both techniques have the potential to markedly increase the effectiveness of radiotherapy through more exact dose administration and minimization of undesired radiation dosage to healthy tissue by decreasing the necessary safety margins.


 
Computed tomography based Method


In order to carry out a computer tomography (CT) scan in the treatment room when the patient is in position, a CT attachment called a Cone-Beam CT is placed on the radiation unit. A cone-beam CT is available in Mannheim (cf. Fig. 1).

 


Figure 1: Treatment machine (linac) with Cone beam CT



With the imaging system directly mounted on the radiation unit, a 3D image of the target region inside the patient can be generated within a minute with a single gantry rotation. This image shows the actual state inside the patient and can be superimposed on images of the nominal state (for example from the radiation dosage plan) for comparison and used to determine how the patient should be repositioned if necessary (Fig. 2).
 


Figure 2:  Superimposed image combining Cone-beam and treatment planning CT


Ultrasound based Method

 


Another method is the daily localization, positioning, and monitoring of the target (e.g. prostate) using stereotactic ultrasound (US). Fig.3 shows the 4D ultrasound imaging system (Clarity, Elekta AB, Sweden) which is currently used in our clinic.




Figure 3: Elekta Clarity System for stereotactic ultrasound based patient positioning



In this method, the position and shape of the target and at-risk anatomic structures (for example, the prostate, seminal vesicles, rectum, and bladder) are transferred from the radiation plan, where they are defined on the basis of an initial US scan, CT and/or MR scans, onto the 4D US imaging device. To enable superimposition of the localization US images in treatment position inside the treatment room onto the reference US image sets acquired during the planning CT, the US system is calibrated to the same coordinate system as the CT and the treatment room.

In just a few minutes (under five minutes) the nominal position of the target organ, for example the prostate, can be compared daily with the actual position. When necessary, the position of the patient can be immediately corrected. The prostate motion can be monitored additionally even during the treatment using this system to see if the organ moves while the treatment beam is on.

The Clarity Ultrasound device shows technicians how far the treatment table (and with it the patient) needs to be shifted along the three primary axes (right/left, up/down, and in the direction of the head/feet) in order to position the prostate exactly in the irradiation field. Ideally, repeatedly measuring and checking the position of the target organ allows the patient to be correctly positioned. Because the method is based on ultrasound, there is no additional exposure with ionization radiation.

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last modified: 03-Jun-15
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