Abstract
PurposeTo investigate the improvement of combined monoscopic/stereoscopic prostate motion monitoring with room‐mounted dual x‐ray systems by adopting patient specific methods.MethodsThe linac couch was used as a motion stage to simulate 40 highly dynamic real patient prostate trajectories. For each trajectory, 40 s pretreatment and 120 s treatment periods were extracted to represent a typical treatment fraction. Motion was monitored via continuous stereoscopic x‐ray imaging of a single gold fiducial and images were retrospectively divided into periods of stereoscopic and monoscopic imaging to simulate periodic blocking of the room‐mounted system by the gantry during arc‐based therapy. The accuracy of the combined motion monitoring was assessed by comparison with the linac couch log files. To estimate 3‐D marker position during monoscopic imaging, the use of population statistics was compared to both maximum likelihood estimation and stereoscopic localization based estimation of individualized prostate probability density functions (PDFs) from the pretreatment period. The inclusion of intrafraction updating was compared to pretreatment initialization alone.ResultsCombined mono/stereoscopic localization was successfully implemented. During the transitions from stereoscopic to monoscopic imaging, fiducial localization exhibits sharp discontinuities when population PDFs were employed. Patient specific PDFs successfully reduced the localization error when estimated from stereoscopic localizations, whereas maximum likelihood estimation (MLE) was too unstable in the room‐mounted geometry. Intrafraction stereoscopic updating provided further increases in accuracy. Residual error tended to decrease throughout the treatment fraction, as the patient‐specific PDFs became more refined.ConclusionsThis is the first demonstration of toggled monoscopic/stereoscopic localization using room‐mounted dual x‐ray imagers, enabling continuous intrafraction motion monitoring for these systems. We showed that both pretreatment individualization and intrafraction updating should be used to provide the most accurate motion monitoring.
Highlights
In modern image guided stereotactic radiation therapy for the prostate, the position of the target may be confirmed at the start of each treatment fraction using a combination of planar imaging and conebeam CT
maximum likelihood estimation (MLE) sometimes produced suitable probability density functions (PDFs) parameters for motion monitoring, there were a number of cases in which large errors (i.e., >5 mm) were incurred, as evidenced by the large range between the median and 75th percentile for the mean and maximum errors using MLE (Fig. 4)
We demonstrated that individualized PDFs can be used to produce more accurate localization during the monoscopic periods, reducing the discontinuities observed during stereo/mono transitions using population averaged PDFs
Summary
In modern image guided stereotactic radiation therapy for the prostate, the position of the target may be confirmed at the start of each treatment fraction using a combination of planar imaging and conebeam CT. Intrafraction prostate motion of 1 cm or more is not uncommon,[1,2] requiring relatively large margins to ensure adequate target coverage,[3] especially in hypofractionated settings.[4,5,6] Alternatively, intrafraction monitoring can be used to gate treatment or track prostate motion, in order to ensure accurate dose delivery, for instance using intrafraction x-ray imaging[7,8,9,10,11,12,13] or electromagnetic transponders.[1,2,3,14,15] Continuous monitoring during treatment is possible using room-mounted stereoscopic x-ray systems, but the treatment head periodically blocks the x-ray sources or detectors, for example, as it rotates during a volumetric modulated arc therapy (VMAT) treatment.[16] quad-x-ray systems have been demonstrated to overcome this limitation,[7] the geometry of a typical room-mounted x-ray system is such that stereoscopic imaging is only available for approximately 50–60° of the 360° gantry rotation (see Fig. 1). For the remaining gantry angles only monoscopic imaging is possible, using either the tube on the right side (red shaded regions) or left side (green shaded regions) of the room
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