Room: Exhibit Hall
Purpose: Inverse optimization has allowed for increased control of dose distributions, in terms of both conformality and homogeneity. In VMAT plans, hotspots are frequently observed in the PTV periphery area outside the GTV. In this study, we present a planning optimization strategy to reduce peripheral hotspots without compromising PTV coverage and OAR doses.
Methods: Ten intracranial patients were planned in Eclipse using VMAT with AcurosXB for dose calculation. Three strategies were investigated, setting separate target optimization constraints to: (1) the PTV, and GTV (with a lower constraint higher than Rx), (2) a â€˜PTV-GTVâ€™ ring structure with tight upper/lower constraints and GTV, and (3) the PTV, PTV-GTV, and GTV. All plans were normalized to achieve the same PTV coverage per patient. Dose thresholds ranging from 103%-105% were used to create high-dose volume (HDV) structures >0.5cc across all plans. Strategies 2 and 3 were compared to strategy 1 by the magnitude and location of HDVs, minimum/maximum target dose, total MU, and dose changes to relevant critical structures.
Results: The global hotspot was located in the periphery of the PTV outside the GTV in all ten patients using strategy 1. Using a PTV-GTV structure reduced peripheral HDVs in all ten patients by an average of 80.4% (strategy 2) and 85.5% (strategy 3), and absolute HDVs by 27.5% and 64.6%, respectively. Ring-based strategies moved the global max hotspot inside the GTV in four patients, and closer to the GTV in the remaining six. PTV min/max doses changed on average by +0.9% and -0.6% with the ring-based strategies. MU increased by 2.3% and 7%, respectively. OAR doses changed by <1% in magnitude.
Conclusion: Use of a PTV-GTV ring structure during plan optimization effectively reduces peripheral PTV hotspots for intracranial lesions. Application of this strategy for other treatment sites is under investigation.