Evaluation of intra‐fractional target displacement by patient motion during a single‐isocenter multi‐target stereotactic radiation therapy for brain metastases

Background Single‐isocenter multi‐target volumetric modulated arc therapy (SIMT‐VMAT) has been implemented widely in fractionated stereotactic radiosurgery (fSRS) to treat brain metastases. The impact of rotational intra‐fractional patient motion (IFPM) is influenced by the distance between the geometric target's center and the isocenter (DTI). Purpose We hypothesized that IFPM's impact on each target would increase with greater DTI during fSRS. Therefore, we aimed to estimate the intra‐fractional target displacement (IFTD), which represents each target's displacement caused by translational and rotational components of IFPM. Methods In this study, we involved 35 patients with 2–13 brain metastases, all of whom had previously undergone SIMT‐VMAT fSRS. All patients were immobilized using a clamshell‐style device, with 28 using a biteplate. Cone beam computed tomography (CBCT) images were obtained at the same imaging center before and after treatment. The IFPM was calculated using both CBCT datasets. The IFTD was determined by comparing the planned target coordinates with the actual coordinates while factoring in IFPM. Results We evaluated 136 targets. The mean IFTD was 0.38 mm (95% confidence interval [CI]: 0.37–0.40 mm) with the biteplate and 0.65 mm (95% CI: 0.59–0.71 mm) without it. A very weak positive correlation was observed between DTI and IFTD despite the immobilization method. This correlation indicates that the distance dependence of IFTD is nearly negligible. Conclusion The findings showed that the impact of IFPM on each target demonstrated minimal dependence on the DTI. Displacement was relatively consistent regardless of the target location. In addition, the use of a biteplate was suggested to potentially reduce these effects.