Implanted Fiducial Markers for Prostate Localization in Radiation Therapy: How Many Markers to Use?
The position and orientation of a rigid body can be defined by specifying the coordinates of three points. Unfortunately, the prostate is not a rigid body, and therefore the orientation information obtained from three fiducials implanted in the prostate is not a reliable measure of its orientation. Fortunately, however, the error in treatment margins resulting from changes in prostate orientation is only about 1 mm, and is therefore within the margins that need to be used in any case to compensate for organ deformation. Since complete and accurate position information can be obtained just as well from a single fiducial, there is really no reason to implant more than one fiducial.
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Fiducial Marker Visibility Requirements for Optimal Use in Target Localization for IGRT
Marker optimization for visibility in different imaging modalities has different, sometimes even conflicting, requirements. For instance, gold seeds, which are designed for optimal visibility in x-ray imaging, produce problematic star artifacts in CT. Fiducial markers for use in radiation therapy should be optimized for use in x-ray, CT, and MRI in order not to interfere with any of these modalities that are commonly used in cancer diagnosis and radiation therapy planning and monitoring.
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Patient Positioning and Position Verification in Prostate Radiation Therapy - non-therapeutic radiation dose to the patient
This white paper reviews the available options for high accuracy patient positioning and position verification in prostate radiation therapy and discusses their advantages and disadvantages. Portal imaging, kilovolt x-ray imaging, and cone beam CT all provide accurate initial position verification, but none of them can provide verification throughout treatment without exposing the patient to excessive non-therapeutic radiation doses. Real time fiducial tracking can provide position verification throughout treatment without increasing the non-therapeutic radiation dose.
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