ESTRO 2021 Abstract Book

S1625

ESTRO 2021

Figure 1: Diagram of HyperArc™ arc geometries. Results

HyperArc™ plans meeting local OAR constraints and plan quality tolerances were created for all 8 patients. The Conformity Index was equal to or lower than the RapidArc™ VMAT plan in all cases. However, normalising the HyperArc™ plans to deliver 80% dose to 99% of the PTV improved the visual conformity. The D 0.1cc to the PTV was below 110% for all plans except one brain metastasis plan, supporting a change in local protocol to restrict hotspots to 110% for benign targets. There was no discernible difference in plan quality using reduced 4 half-arc geometry compared to full 5 half-arc geometry.

Figure 2: Table comparing dosimetric and volumetric results for normalised plans. All PDIP analyses passed at 3%/2mm, with a low dose threshold of 20% and 95% gamma passing threshold. Only one plan failed Octavius analysis using the same criteria as PDIP, and passed using a reduced VOI to discount low dose gamma failures far from the PTV. All but one plan passed DoseCHECK™ analysis at 3%/1mm, with a 20% low dose threshold and 95% gamma passing rate. As the failed plan had PTV abutting bone, differences in dose modelling across the density interface reasonably increased the failure rate. This plan passed after recalculating at 4%/1mm criteria. Conclusion HyperArc™ is suitable for single-target SRS treatments for brain metastases, meningiomas, and acoustic neuromas, using reduced four arc geometries and normalising the prescription dose to 99% of the PTV as standard. The results justify restricting hotspots to 110% for benign targets. DoseCHECK™, PDIP, and Octavius analysis verify dosimetric accuracy and plan deliverability.

PO-1905 Combined proton-photon treatment for breast cancer using a fixed proton beamline L. Marc 1 , S. Fabiano 1 , N. Wahl 2 , C. Linsenmeier 1 , A. Lomax 3,4 , J. Unkelbach 1

1 University Hospital Zurich, Radiation Oncology, Zurich, Switzerland; 2 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany; 3 Paul Scherrer Institute, Center for Proton Therapy, Villigen, Switzerland; 4 ETH Zurich, Physics, Zurich, Switzerland