Room: Exhibi Hall | Forum 6
Purpose: To investigate the accuracy of Intrafraction Cone Beam Computed Tomography (IF-CBCT) in detecting intrafraction target motion.
Methods: An Elekta linear accelerator with XVI imaging system was used for this study. A CIRS dynamic thorax phantom with an in-house developed insert consists of three radiopaque markers and a head section of an anthrophomorphic phantom was used to simulate the intrafraction target and patient motion in prostate and brain treatments respectively. The IF-CBCT images of the phantoms were acquired during the hypofractionated radiotherapy delivery using 6MV VMAT arcs. Three types of intrafraction motion (a)Static offset, (b)Continuous drift and (c)Transient excursions ranging from 1mm to 8mm were introduced to the phantom during treatment delivery. The onset of motion in each scenario was set at four different time points, at the start and at 25%, 50% and 75% completion of treatment. The IF-CBCT images were registered with the reference planning CT by manual and seed and bone auto-registration methods available in XVI to quantify the position deviations.
Results: The static offset introduced at the start of treatment was identified by IF-CBCT images in both prostate and brain treatments. Both the manual and auto registration methods agreed with the set offsets within Â±1mm. The static offset and continuous drift introduced at 25% 50% and 75% of the treatment completion demonstrated the presence of motion during image acquisition. The position offsets calculated using these IF-CBCT images showed major difference, up to 4mm for motion at 25% and up to 7mm for motion set at 75% for the offset magnitude of 8mm.
Conclusion: The IF-CBCT images demonstrated the presence of intrafraction motion qualitatively, however the position offsets calculated using these images showed major difference from the actual offsets and necessitate the additional CBCT to quantify the positional offsets accurately.
Not Applicable / None Entered.