Feasibility Study of a New Ring-Gantry Linac for Single Vertebral Segment SBRT

Abstract

The novel, ring-gantry linear accelerator machine allows only translational and no rotational couch corrections. This study aims to compare treatment plan quality for single vertebral segment SBRT using the ring-gantry linear accelerator and the medical linear accelerator machines. In addition, this study investigates the effect of patient rotational setup errors on the delivered plan quality. We analyzed 20 patients with single-level spine metastasis, located between T7 and L5 near the spinal cord. The median tumor volume was 64.3 cm3 (17.9-139.1 cm3) and the median Z-axis tumor diameter was 3.1cm (2-5cm). The prescription doses were 12-16 Gy in 1 fraction or 18-24Gy in 3 fractions. All patients were originally treated in the TB machine with 2.5 mm leaf width, and replanned for the Halcyon machine with 5mm effective MLC leaf width. Offline CBCT fusion from 50 SBRT spine treatments showed an average rotation of 0.63, 1.19 and 0.79 degree in pitch, yaw and roll. Rotational error in roll was not considered in this study since the original TB plans used coplanar volumetric modulated arc therapy (VMAT) technique. Four CT image datasets were generated with different rotational combinations of pitch and yaw for 1 degree in Velocity to simulate different scenarios of patient setup errors. We recalculated the Halcyon plans on rotated images. Dosimetric plan quality was evaluated for tumor coverage, conformity index (CI), maximum dose to the cord, and volume of the cord receiving 8, 10, and 12 Gy. Paired student’s t-test was performed with a significance level of 0.01. The commercially available treatment planning system based VMAT plans for Halcyon achieved similar target coverage (92.3 ± 3.0% vs. 91.2 ± 5.5%, p=0.21), and CI (1.0 ± 0.1 vs. 1.1 ± 0.2, p=0.09) compared to TB plans. Maximum dose to the spinal cord was comparable (11.1 ± 2.8 Gy vs. 12.1 ± 4.2 Gy, p =0.05). So were the V8, V10 and V12 for the cord. The dosimetric influence of patient rotational setup error was statistically insignificant for rotations of up to 1 degree pitch/yaw (with similar target coverage, CI, max cord dose and V8, 10, 12 for cord). The total MUs for Halcyon (4739 ± 1522) were slightly less than TB (5412 ± 2057). VMAT plans for single spine metastasis were dosimetrically comparable to TB plans. As long as the patient rotation is within 1 degree in the pitch and yaw directions, the dosimetric effect was insignificant. Halcyon is an alternative to TB for the treatment of single spinal level metastases, but users need to be cautious about the patient rotational setup error.Abstract 3789; Table 1Comparison of Dosimetric indices on 1 degree rotated imageVariableTilt 01A1B1C1DConformity Index (CI)1.09±0.241.10±0.241.10±0.241.10±0.241.10±0.24PTV Coverage (%)91.2±5.4691.0±5.5591.0±5.5791.2±5.3791.1±5.64Max Cord (Gy)12.1±4.1712.0±4.2212.1±4.1812.1±4.2412.1±4.16Spinal Cord V8 (cc)2.57±2.102.52±2.082.53±2.072.52±2.072.51±2.06Pitch / Yaw: -1°/-1° (1A), -1°/+1° (1B), +1°/-1° (1C), +1°/+1° (1D); All P values>0.01 (n=20, paired t-test, 2 tails) Open table in a new tab