Room: AAPM ePoster Library
Purpose: To establish a correlation among target coverage, motion amplitude, and the number of volumetric paintings through measurements, and to provide a novel reference on informed decision making for lung proton PBS treatment.
Methods: A CIRS thoracic dynamic phantom with center-to-center superior-inferior target motion of 5, 10, 15 and 20mm were scanned using 4DCT clinical protocol. Plans were generated on the averaged CT based on ITV using a 3-field single-field-optimization technique. An in-house 4D motion evaluation tool was used to quantify the target motion based on voxel WET deviation among phases. Carefully calibrated EBT3 films were placed inside of the target to measure the dose. A novel error subtraction method was developed to reduce uncertainties from inherent TPS modeling and CT calibration to approach the ground truth of the dose interplay. The gamma passing rate (GPR) (3%/4mm,10% threshold), 2D dose profiles, and mean dose were used as metrics to characterize the target dose interplay.
Results: Dose degradation has a clear correlation with the motion amplitude and the number of paintings. The measurements show a wider-blurred dose pattern and higher dose heterogeneity compared with the static plans for larger motion and fewer paintings. For 5mm motion, 3-paintings achieve a GPR >98%, while 5- and 7-paintings iteratively improve the GPR to 99%. For 10mm motion, 3-paintings provide a GPR of 94%, which increases to 95% and 97% for 5- and 7-paintings, respectively. For 15mm motion, 3-paintings only show a GPR of 85%, and 5- and 7-paintings boost the GPR to 92% and 94%. Furthermore, for 20mm motion, 3-,5- and 7-paintings only produce GPRs of 75%, 85%, and 89%.
Conclusion: The experimentally verified correlation can predict target coverage regarding the motion amplitude and the number of volumetric paintings. The study indicates that large motion requires more paintings (>7)/fraction to maintain treatment quality.