(Sunday, 7/14/2019) 3:30 PM - 4:00 PM
Room: Exhibit Hall | Forum 5
Purpose: On-board 4D-CBCT imaging integrated with linac was commissioned for gated delivery. To properly implement the technology, 4D-CBCT image acquisition was tested and its images were registered with 4D-CT images using a commercial motion phantom.
Methods: Quasar™ motion phantom was used for absolute dosimetry in a gated treatment of SBRT plan for a thoracic lesion. Small ionization chamber was inserted into a plug. 50Gy was prescribed in five fractions using 2 coplanar VMAT beams. The plan was delivered using Varian TrueBeam™ linac with real-time position management system for respiratory gating (40-60% phases in 4D-CT scan). Prior to the delivery, 4D-CBCT was acquired and registered with 4D-CT images for 40-60% phases. The selected breathing pattern was 12 breaths per minute (BPM) with ±1cm motion.
Results: The ionization chamber measured dose was close to the calculated dose from treatment planning system within 0.3% relative difference. 4D-CT images with improper Cine duration time (CDT) was not matched with 4D-CBCT with respect to different breathing phases. For instance, if CDT in 4D-CT scan is chosen as an average of breathing period plus 1.5 second as gantry rotation, the breathing phases in 4D-CT was shifted by 10% compared to the 4D-CBCT, i.e. 40-60% of 4D-CBCT corresponding to 50-70% of 4D-CT. CDT as average breathing period plus 0.8 second of gantry rotation was found to match breathing phases in 4D-CT in comparison with those in 4D-CBCT. Regardless of fast breathing (20 BPM) or slow breathing (10 BPM), the 40-60% phases of 4D-CBCT was the same as those of 4D-CT. In contrast, change of breathing amplitude could affect the size of object differently between 4D-CT and 4D-CBCT.
Conclusion: To properly implement 4D-CBCT in the clinic, the phases of 4D-CBCT should be evaluated by comparing them to those of 4D-CT and appropriate CDT should be selected.
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