Dosimetric Evaluation of VMAT Grid Therapy for Bulky Tumors Using Generalized Equivalent Uniform Dose (gEUD) and Normal Tissue Objective (NTO) Optimization
Abstract
Purpose
Spatially fractionated radiation therapy, specifically grid radiation therapy(GridRT),is a treatment option for bulky tumors.VMAT GridRT utilizes modulated arcs to create the grid of spherical targets within the tumor.This study evaluates optimization parameters to determine the ideal strategy for VMAT GridRT planning.
Methods
In this retrospective planning study,a cohort of 10 patients was studied. Mean GTV volume was 509.6±281.3cm3.Plans utilized 5-15 spheres of 1.5cm diameter. Spheres were placed with the previous studies. A prescription of V50%=20Gy to each sphere in a single fraction was used. 2 distinct VMAT GridRT plans using 6MV-FFF with 2 full arcs were designed and optimized.One plan was optimized with physical dose-volume objectives(D-V) without normal tissue objectives(NTO),and the other plan was optimized with generalized equivalent uniform dose(gEUD) and manually built-in NTO(gEUD+NTOmanual) in Eclipse™(V16.1).A comprehensive comparative evaluation was performed to assess dosimetric quality.Metrics evaluated included the Dmean and dose gradient(D10%Rx/D90%Rx) for the GTV and all sphere targets(z_Sphere all),and Dmax for all inner,middle and outer rings(z_IC, z_MC and z_OC) and normal tissues(body-spheres).The peak‐to‐edge dose ratio(PEDR) and peak‐to‐valley dose ratio(PVDR) were evaluated.Differences were assessed for statistical significance(p<0.05) by use of Student's t-test.
Results
No significant dosimetric difference in Dmean and dose gradient was seen for the GTV or z_Sphere all.However, gEUD+NTOmanual plans provided more consistent and conformal dose for each sphere target. gEUD+NTOmanual plans showed significantly lower Dmax in all rings and the body-spheres volume compared to D-V.The PVDR and PEDR did not change significantly (p~0.2).Optimization time was approximately 5min for gEUD+NTOmanual optimizations and increased to at least 10min for D-V to achieve similar target Dmean.
Conclusion
VMAT GridRT for bulky tumors is an emerging treatment technique with challenging planning requirements.Utilization of a single gEUD objective per sphere combined with NTO,can simplify optimization setup and reduce optimization time while preserving and/or improving plan quality compared to traditional physical optimization methods.