Room: AAPM ePoster Library
Purpose: To develop and commission a workflow for free breathing amplitude gated treatments utilizing the imaging features available with the TrueBeam 2.7 system.
Methods: A 4DCT scan is acquired at time of simulation and target motion is evaluated. The phases over which motion is minimized are determined and used to reconstruct a phase subset average CT for planning. During planning a tracking structure is contoured to indicate the position of the target during treatment delivery, which is typically the target/relevant surrogate ITV over the planning phases. For pre-treatment imaging, a 4DCBCT scan is acquired and an average CBCT is reconstructed over the planning phases for an initial match to the planning CT. The 4DCBCT movie loop is reviewed to assess target motion. Fluoroscopy images are then obtained at a minimum of one angle to set the amplitude gate thresholds that correspond to when the target is in the planning tracking structure. The final imaging confirmation prior to treatment is an amplitude gated CBCT to verify the amplitude gate and patient positioning. An amplitude gated treatment is then delivered. This workflow was commissioned using an in-house motion phantom capable of programmable superior/inferior motion.
Results: End to end testing showed agreement between 4DCT and 4DCBCT relative target positions throughout the respiratory cycle, thus verifying the accuracy of 4DCBCT phase binning. Film measurements compared to the planned dose distribution showed strong agreement for both regular and irregular breathing traces. The technique was found to be reproducible and robust to changes in upper gate threshold of 20%.
Conclusion: The workflow developed in this work utilizes the advanced imaging features available with TrueBeam 2.7 software to ensure the accurate delivery of free breathing amplitude gated treatments. It is efficient and robust to changes in breathing trace.