Evaluation of HyperArcTM using film and portal dosimetry quality assurance

Abstract HyperArc™ is a stereotactic radiotherapy modality designed for targeting multiple brain metastases using a single isocenter with multiple non-coplanar arcs. The purpose of this study was to assess the efficacy of two patient specific quality assurance methods, film and the Varian Portal Dosimetry System with Varian’s HyperArc™ technique and raise important considerations in the customisation of patient specific quality assurance to accommodate HyperArc™ delivery. Assessment criteria included gamma analysis and mean dose at full width half maximum. The minimum met size, maximum off-axis distance and suitable energy were identified and validated. Patient specific quality assurance procedures were applied to a range of clinically relevant brain met plans. Initial investigation into energy selection showed no significant differences in gamma pass rates using 6MV, 6MV FFF or 10MV FFF for met sizes greater than 15 mm diameter at the isocenter. Gamma pass rates (2%/2mm) for 15 mm mets at the isocenter for all energies were greater than 96.0% for portal dosimetry and greater than 98.7% for film. Fields of size 15 mm placed at various distances (10–70 mm) from the isocenter resulted in a maximum mean dose difference of 1.5% between film and planned. Clinically relevant plans resulted in maximum mean dose difference for selected mets of 1.0% between film and plan and maximum point dose difference of 2.9% between portal dose and plan. Portal dose image prediction was found to be a quick and convenient quality assurance tool for mets larger than 15 mm near the isocenter but provided diminished geometrical relevance for off-axis mets. Film QA required exacting procedures but offered the ability to assess the accuracy of geometrical targeting for off-axis mets and provided dosimetric accuracy for mets to well below 15 mm diameter.