A Simple HU-Override Strategy to Improve Robustness of Acuros Xb Optimization In Targets Overlapping Air
Abstract
Purpose
To evaluate whether a low-density HU-override strategy can reduce Acuros XB (AXB) optimizer over-compensation when air volumes overlap the planning target volume (PTV), while maintaining dose coverage and robustness at the air–tissue interface and reducing hotspots when air is replaced by tissue.
Methods
Patients with low-density/air within or adjacent to the PTV were identified across three disease sites: prostate (n=6), HN(n=6), and abdominal/pelvic cases (n=6). An HU-override structure was defined by thresholding HU < −730 in the air cavity within 5mm of PTV. Three plans were generated per patient: AXB optimized without HU override (Plan A) and AXB optimized with the HU-override assigned to the lung and calculated with the HU override removed (Plan B). Evaluation metrics included interface tissue coverage, interface tissue coverage with simulated setup errors shifting air fluence into PTV tissue, and maximum dose after reassigning the HU-override structure to “muscle” to simulate tissue replacing air.
Results
Percent change in PTV_Interface coverage corresponding to V98% for Plan B relative to Plan A: -0.7% +/-1.1% and for the robustness shifted plan for Plan B relative to Plan A: -2.2% +/-2.7%. Change in Dmax in air/tissue replacement for Plan A relative to Plan B: 5.9% +/-7.2% with deviations up to 19.7% higher for Plan A.
Conclusion
Optimized plans achieved high coverage, with PTV_Interface achieving equivalent coverage between Plan A and Plan B. Simulated setup errors showed equivalent coverage of the PTV_Interface. However, robustness testing demonstrated that AXB optimization without override could produce substantial hotspots when air was replaced by tissue, whereas Plan B would minimize this effect due to the reduction of fluence in the air portion of the PTV. The lung-assigned HU-override strategy reduced the magnitude of these hotspots while maintaining interface coverage.