(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: It is important to have the most accurate dose representation in thoracic SBRT plans because of higher dose gradient near critical structures such as spinal cord and presence of tissue heterogeneity. We investigated dosimetric effects associated with changing grid-sizes for thoracic-spine SBRT plans using IMRT and VMAT techniques utilizing Acuros-XB calculation algorithm.
Methods: Fifteen patients treated with SBRT for thoracic-spine were selected, 5 planned with IMRT and 10 with VMAT. VMAT plans consisted of 2-3 full arcs while IMRT plans consisted of 8-12 static fields. All clinical plans were calculated using the Anisotropic-Analytical-Algorithm(AAA) and were then recalculated using Acuros-XB(AXB) using grid sizes of 2.5mm,1.5mm, and 1mm using Eclipse-TPS.Various Dosimetric parameters such as V90,D95,Dmax,Dmin,Dmean and Conformality-Index(CI) for PTV and maximum, mean and Dv% dose-parameters for OARs were compared.
Results: PTV maximum deviations between grid-sizes of 2.5mm Vs 1.5mm were 1.2%,2.3%,7.5%,8.4%,1.3%, 0.2%; between 2.5mm Vs 1mm were 1.9%,2.8%,9.2%,4.7%,1.5%,0.2%; between 1.5mm Vs 1mm were 0.8%,0.9%, 2.6%,1.3%,0.4%,0.2 for V90,D95,Dmax,Dmin,Dmean and Conformality-Index(CI) respectively.Cord maximum deviations between grid-sizes of 2.5mm vs 1.5mm were 7.0%,2.4%,3.7%,11.7%; between 2.5mm Vs 1mm were 9.5%,3.8%,4.2%,13.8%; between 1.5mm vs 1mm were 3.7%,1.5%,0.6%,2.1% for Dmax, Dmin, Dmean, D10% respectively. The Heart and esophagus dose parameters and plan calculation time were also compared for various grid-sizes.
Conclusion: 1mm grid-size resulted in the greatest increase in PTV coverage and the greatest decrease in dose to organs at risk. Without the use of a GPU-calculation based server, 1mm grid-size seems practically unacceptable due to its significantly increased calculation time. The 1.5mm calculation grid-size proved to have comparable results to the 1mm grid-size with much shorter calculation times. The 1.5mm provides better PTV coverage as well as OAR sparing compared to the 2.5 mm grid-size. We recommend using 1.5mm grid-size to best compromise between dose accuracy and calculation time for thoracic-Spine SBRT plans using Acuros-XB .
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