Retrospective quality metrics review of stereotactic radiosurgery plans treating multiple targets using single-isocenter volumetric modulated arc therapy.

Yunfeng Cui,Fang-Fang Yin,Jiahan Zhang,John P Kirkpatrick,Hao Gao

doi:10.1002/acm2.12869

Abstract

PurposeTo characterize key plan quality metrics in multi‐target stereotactic radiosurgery (SRS) plans treated using single‐isocenter volumetric modulated arc therapy (VMAT) in comparison to dynamic conformal arc (DCA) plans treating single target. To investigate the feasibility of quality improvement in VMAT planning based on previous planning knowledge.Materials and methods97 VMAT plans of multi‐target and 156 DCA plans of single‐target treated in 2017 at a single institution were reviewed. A total of 605 targets were treated with these SRS plans. The prescription dose was normalized to 20 Gy in all plans for this analysis. Two plan quality metrics, target conformity index (CI) and normal tissue volume receiving more than 12 Gy (V12Gy), were calculated for each target. The distribution of V12Gy per target was plotted as a function of the target volume. For multi‐target VMAT plans, the number of targets being treated in the same plan and the distance between targets were calculated to evaluate their impact on V12Gy. VMAT plans that had a large deviation of V12Gy from the average level were re‐optimized to determine the possibility of reducing the variation of V12Gy in VMAT planning.ResultsConformity index of multi‐target VMAT plans were lower than that of DCA plans while the mean values of 12 Gy were comparable. The V12Gy for a target in VMAT plan did not show apparent dependence on the total number of targets or the distance between targets. The distribution of V12Gy exhibited a larger variation in VMAT plans compared to DCA plans. Re‐optimization of outlier plans reduced V12 Gy by 33.9% and resulted in the V12Gy distribution in VMAT plans more closely resembling that of DCA plans.ConclusionThe benchmark data on key plan quality metrics were established for single‐isocenter multi‐target SRS planning. It is feasible to use this knowledge to guide VMAT planning and reduce high V12Gy outliers.

Highlights

Brain metastases occur in many cancer patients with different incident rates depending on cancer type.[1,2] Patients who develop brain metastases are associated with shorter life expectancy.[3]
All dynamic conformal arc (DCA) plans were generated in iPlan treatment planning system and all volumetric modulated arc therapy (VMAT) plans were generated in Eclipse treatment planning system for linear accelerators equipped with Varian HD120 multi‐leaf collimator (MLC) (Varian Medical Systems, Palo Alto, CA)
The p values resulting from Mann‐Whitney tests comparing DCA and VMAT groups were included in the table

Summary

Introduction

Brain metastases occur in many cancer patients with different incident rates depending on cancer type.[1,2] Patients who develop brain metastases are associated with shorter life expectancy.[3]. Single‐isocenter volumetric modulated arc therapy (VMAT) technique treating multiple lesions at the same time made the multi‐target SRS more practical by dramatically reducing the treatment time compared to treating each target separately.[9,10,11,12,13] With the ability of beam modulation and inverse optimization, the single‐isocenter multi‐target (SIMT) VMAT plan can achieve good dose conformity to targets compared with other SRS treatment techniques.[14,15] On the other hand, because of the complexity of VMAT planning the plan quality metrics across different planning systems and planners could be less consistent.[16,17,18] There is a concern especially for the dose fall‐off gradient and low dose spread in SIMT VMAT plan due to the potential large opening of multi‐leaf collimator (MLC) causing irradiation of normal tissue in between targets.[19,20,21] Among those dosimetric metrics, normal brain volume receiving more than 12 Gy (V12Gy) is a biomarker that has been identified to correlate with normal tissue necrosis.[7,22,23] A benchmarking data on SRS plan dosimetric metrics will provide the planners guidance about what can be achieved for specific cases.[24,25,26]

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