Normal Brain Dose and Treatment Efficiency of Coplanar-Only IMRT/VMAT plans for Glioblastoma Multiforme using a Novel Ring Gantry Linac Delivery System

Abstract

A novel ring gantry linac delivery system (RDS) with a 6MV flattening filter free (FFF) photon beam, a fast gantry rotation, and a jawless, fast dual-layer multileaf collimator (MLC) is now available commercially. The new RDS provides only the coplanar beam arrangements for IMRT or VMAT to treat glioblastoma multiforme (GBM). The purpose of this study was to compare the normal brain dose and delivery efficiency of treatment plans using the RDS with those obtained from the clinically treated plans using conventional linacs. Ten clinical IMRT or VMAT plans (4 coplanar and 6 non-coplanar, 46 Gy/23 fx) used previously for GBM treatment were retrospectively selected. The corresponding coplanar RDS plans were created with the same technique and number of beams/arcs using a treatment planning system. All RDS plans were optimized and normalized such that target coverages, average dose (Dmean) to PTV1-46Gy and Conformity index (CI) were comparable between the RDS plans and clinical plans. The RDS plans were also optimized to meet the same OAR dose constraints as the clinical plans. The R50 and D2cm from the isocenter in transverse and superior-inferior directions were compared. The normal brain dose (Brain minus PTV1) was evaluated with V10Gy and V30Gy. Treatment efficiency of the RDS was compared by recording the delivery time of all beams without including CBCT imaging/registration time. All RDS treatment plans were dosimetrically comparable to clinically treated plans; these also fulfilled the prescribed constraints and priorities: Dmean (47.7 ± 0.6 Gy (RDS) vs. 47.3 ± 0.6 Gy (linacs)) and CI (0.97 ± 0.09 (RDS) vs. 0.96 ± 0.09 (linacs)). When compared with non-coplanar linac plans, the average beam delivery times of the RDS plans are significantly reduced depending on the number of arcs/angles (2.1 ± 0.5 min (RDS) vs. 9.7 ± 2.9 min (linacs), p=0.007)). The median R50 of the RDS plans is about 7.7 % larger than the median R50 of the clinical linac plans (2.8 ± 0.7 (RDS) vs. 2.6 ± 0.5 (linacs). D2cm of the RDS plans in the transverse orientation is 13.8 % higher than the coplanar clinical linac plans (30.6 ± 3.2 Gy (RDS) vs. 26.9 ± 2.6 Gy (linacs)). It is noteworthy that the transverse D2cm becomes elevated to + 65 % when compared only with the non-coplanar linac plans. The V10Gy and V30Gy of two groups (RDS vs. non-coplanar linacs) are 73.3% vs. 68.9% and 22.9% vs. 21.1%, respectively. Our results show that the new RDS system provides coplanar IMRT/VMAT plans for the GBM with significantly improved treatment efficiency with no collision risk. When using the same technique and # of beams/arcs with non-coplanar linac plans, the R50, V10Gy and V30Gy, and transverse D2cm of the RDS plans become significantly elevated. Proper addition of arcs or static beams can adjust the parameters (data not shown).