Purpose: Fiducial markers are frequently used for image-guided patient setup and similarly can be used during treatment to detect intrafraction motion. We investigate using commercially available periodic kV imaging and automatic marker detection(TrueBeam v2.5) during VMAT prostate treatments to monitor/correct intrafraction motion.
Methods: Ten patients had 2-3 collapsible intraprostatic fiducials. During VMAT treatment, kV images were acquired using on-board imaging every 10sec during treatment. For each image, Varianâ€™s software determined fiducial location relative to planned locations in real-time to monitor for motion >3mm. Motion correction occurred if either two fiducials in a single image or one fiducial in sequential images were out of tolerance. Software-based fiducial identification was visually verified. A phantom mimicking patient composition/geometry with 3 implanted collapsible fiducials allowed evaluating specific aspects of this approach: fiducial-type setting, repeatability and accuracy of software detection.
Results: Intrafractional kV imaging revealed necessary shifts in 23Â±16% of total fractions/patient (212 overall fractions). All patients required at least one shift over a treatment course. Average shift magnitude was 3.4Â±1.1mm (largest shift=8.4mm). On average, shifts added 3.1Â±0.4min to treatment. Fiducial shape/configuration greatly affected accuracy/precision of detection. For patients with three crumpled (n=7) versus one or more linear (n=3) fiducials, the softwareâ€™s ability to correctly identify fiducials in/out of tolerance dropped from 51% to 4%. Phantom studies supported this finding. Additionally, they showed wide variability in the softwareâ€™s ability to accurately detect fiducials across 8 available fiducial-type settings. For a particular setting, however, results were relatively reproducible and precise to ~0.5mm.
Conclusion: Periodic kV imaging during VMAT prostate treatments is a commercially available method, which allows for monitoring/correcting intrafraction motion. Appropriate use requires attention to fiducial type/configuration and identifying optimal software settings. With proper marker configuration and software settings, phantom/patient studies demonstrate that periodic imaging with real-time marker detection helps mitigate effects of intrafraction motion.
Not Applicable / None Entered.