Evaluation of Margin for Intra-Fractional Patient Motion during Single-Isocenter Multi Targets Volumetric Modulated Arc Therapy Stereotactic Radiation Therapy for Brain Metastases Using Actual Target Coordinates

Abstract

<h3>Purpose/Objective(s)</h3> Single-isocenter multi targets volumetric modulated arc therapy (SIMT-VMAT) has been used extensively in stereotactic radiation therapy (SRT) for brain metastases. However, to the best of our knowledge, there is no report on the position displacement of each target by the intra-fractional patient motion (IFPM) during SIMT-VMAT SRT. In particular, the effect of rotational IFPM is assumed to be greater depending on the distance between the center of gravity of the target and the isocenter (DTI). We hypothesized that the margin for IFPM increased depending on the DTI during SIMT-VMAT SRT. Thus, this study aimed to evaluate the margin for the IFPM during SIMT-VMAT SRT by estimating actual target displacement (ATD), which consisted of translational and rotational displacement (RD) for each target by the IFPM. <h3>Materials/Methods</h3> The study included 25 patients (aged 49–83 years; 13 men and 12 women) with 2-13 brain metastases who had previously received SIMT-VMAT SRT, with four fractions in our institution. The total number of targets for all patients was 101 and the DTI range was 14–85 mm. The clamshell-style immobilization device immobilized all patients. Bite-block was used in 20 patients. Cone beam computed tomography (CBCT) images were acquired in pre- and post-treatment within the same imaging center. The IFPM was calculated from both CBCT images. Both ATD and RD were calculated by the difference between the planned target coordinates and the actual target coordinates, which were considered with the IFPM. The Spearman rank correlation coefficient (r_s) was used for the estimation of the correlation of RD with DTI and ATD with DTI. The Wilcoxon-signed rank test was used for comparing ATD, with and without bite-block. Statistical significance was established at <i>P</i> <0.05. <h3>Results</h3> In SIMT-VMAT SRT with bite-block, the median of ATD and RD was 0.37 mm (95% confidence interval (CI): 0.019–0.80 mm) and 0.15 mm (95% CI: 0.016–0.41 mm), respectively. We observed a weak positive correlation in RD with DTI (r_s=0.39, <i>P</i> <0.05) and no correlation in ATD with DTI (r_s=0.035, <i>P</i> =0.53). On the other hand, in SIMT-VMAT SRT without bite-block, the median of ATD and RD was 0.66 mm (95% CI 0.078–1.5 mm) and 0.43 mm (95% CI 0.065–1.0 mm), respectively. We observed a positive correlation in RD with DTI (r_s=0.52, <i>P</i> <0.05) and a weak positive correlation in ATD with DTI (r_s=0.28, <i>P</i> <0.05). There was a significant difference in ATD with and without bite-block (<i>P</i> <0.05). <h3>Conclusion</h3> These results indicate that the margin for the IFPM in SIMT-VMAT SRT with the bite-block would be equal regardless of the DTI. Our results indicated the bite-block increased the accuracy of patient immobilization. Moreover, it was suggested that the margin for the IFPM would be uniformly below 1 mm from the edge of the target.