Session Item

To show HyperArc™ plans have equivalent or improved plan quality compared to RapidArc™ VMAT, for SRS treatment of brain metastases, meningiomas, and acoustic neuromas, with the overall goal of commissioning HyperArc™ treatments for single cranial targets.

8 patients who had previously received non-coplanar RapidArc™ VMAT for single targets on a Varian Novalis Tx linac were replanned using HyperArc™ for treatment on a Varian Edge linac. The patients represented a range of sites within the skull, constituting 4 brain metastases, 2 acoustic neuromas, and 2 meningiomas. 4 plans had isocentres within the central patient protection zone, and were planned with both full arc geometry using the standard 5 half-arcs and reduced arc geometry using 4 half-arcs excluding the central arc as in Figure 1. The other 4 had lateral isocentres, and were planned with reduced arc geometry of 4 half-arcs excluding the contra-lateral arc. 1 brain metastasis and 1 meningioma patient additionally had some overlap of the PTV with brainstem. Plan quality was compared to the previous RapidArc™ VMAT plan. The effects of normalising to deliver 80% dose to 99% of the PTV were additionally investigated. Each HyperArc™ plan was verified by analysis with both PDIP and Octavius, and analysed by DoseCHECK™ independent dose calculation.

Figure 1: Diagram of HyperArc™ arc geometries.

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.

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.