Exhibit Hall | Forum 3
Purpose: Acuros XB in Eclipse could provide superior handling of tissue heterogeneities compared to AAA and behaves differently in low HU lung regions, which calls for updated plan evaluation criteria when transitioning from AAA to Acuros for lung SBRT planning. The study investigates the sensitivity of target dose metrics to lung HU in Acuros lung SBRT plans compared to AAA plans.
Methods: Ten clinical lung SBRT cases planned with dynamic conformal arcs or VMAT were retrospectively selected. For each patient, the lung HU (excluding ITV) was manually overridden to values from -950 to -600 to mimic different lung conditions. The original plan was first calculated with Acuros (dose to water) in Eclipse (version 15.6) using lung HU of -600 and was normalized to cover 95% of PTV with prescription dose. Then the same MUs were used for dose calculation with different lung HU values. Target DVHs were recorded and compared. The same process was repeated with AAA.
Results: The Acuros plans’ target coverages decreased with decreasing lung HU values, especially under -850HU. Compared to -600HU plans, the average PTV mean doses were 33.4%(10.4%), 20.6%(10.3%), 13.0%(7.8%), 8.1%(5.4 %), and 2.5%(1.9%) of prescription dose lower for -950HU, -900HU, -850HU, -800HU, and -700HU Acuros(AAA) plans, respectively. Low lung HU resulted in larger dose heterogeneity inside PTV, with higher impact on Acuros plans compared to AAA. The average PTV heterogeneity index (D5%/D95%) was 1.61(1.21), 1.31(1.19), and 1.21(1.17) for -950HU, -900HU, and -850HU Acuros(AAA) plans.
Conclusion: Lower target coverages and higher PTV dose heterogeneities are expected for patients with low CT numbers of the lungs when using Acuros for lung SBRT planning. Acuros dose calculation exhibited high sensitivity to small CT number changes (50HU) compared to AAA, especially in the low-density range, which necessitates accurate HU calibration of CT simulators.