Purpose. Accurate tumor localization for image-guided liver stereotactic body radiation therapy (SBRT) is challenging due to respiratory motion and poor tumor visibility on conventional x-ray based images. Novel integrated MRI and radiotherapy systems enable direct in-room tumor visualization, potentially increasing treatment accuracy. As these systems currently do not provide a 4D image-guided radiotherapy strategy, we developed a 4D-MRI guided liver SBRT workflow and validated all steps for implementation on the Unity MR-linac. Materials and Methods. The proposed workflow consists of five steps: (1) acquisition of a daily 4D-MRI scan, (2) 4D-MRI to mid-position planning-CT rigid tumor registration, (3) calculation of daily tumor midP misalignment, (4) plan adaptation using adapt-to-position (ATP) with segment-weights optimization and (5) adapted plan delivery. The workflow was first validated in a motion phantom, performing regular motion at different baselines (5 to 10 mm) and patient-derived respiratory signals with varying degrees of irregularity. 4D-MRI derived respiratory signals and 4D-MRI to planning CT registrations were compared to the phantom input, and gamma and dose-area-histogram analyses were performed on the delivered dose distributions on film. Additionally, 4D-MRI to CT registration performance was evaluated in patient images using the full-circle method (transitivity analysis). Plan adaption was further analyzed in-silico by creating adapted treatment plans for 15 patients with oligometastatic liver disease. Results. Phantom trajectories could be reliably extracted from 4D-MRI scans and 4D-MRI to CT registration showed submillimeter accuracy. The DAH-analysis demonstrated excellent coverage of the dose evaluation structures GTV and GTV_TD. The median daily rigid 4D-MRI to midP-CT registration precision in patient images was <2 mm. The ATP strategy restored the target dose without increased exposure to the OARs and plan quality was independent from 3D shift distance in the range of 1–26 mm. Conclusions. The proposed 4D-MRI guided strategy showed excellent performance in all workflow tests in preparation of the clinical introduction on the Unity MR-linac.

目的。由于呼吸运动和传统 X 射线图像上的肿瘤可见度差，图像引导的肝脏立体定向放射治疗 (SBRT) 的准确肿瘤定位具有挑战性。新型集成 MRI 和放射治疗系统可实现直接的室内肿瘤可视化，有可能提高治疗准确性。由于这些系统目前不提供 4D 图像引导的放射治疗策略，我们开发了 4D-MRI 引导的肝脏 SBRT 工作流程，并验证了在 Unity MR-linac 上实施的所有步骤。材料和方法. 建议的工作流程包括五个步骤：(1) 获取每日 4D-MRI 扫描，(2) 4D-MRI 到中间位置规划-CT 刚性肿瘤配准，(3) 计算每日肿瘤 midP 错位，(4)使用适应位置 (ATP) 和段权重优化的计划适应和 (5) 适应计划交付。该工作流程首先在运动体模中进行了验证，在不同的基线（5 到 10 毫米）和来自患者的呼吸信号具有不同程度的不规则性时执行规则运动。将 4D-MRI 衍生的呼吸信号和 4D-MRI 计划 CT 配准与体模输入进行比较，并对胶片上传递的剂量分布进行伽马和剂量面积直方图分析。此外，使用全圆方法（传递性分析）在患者图像中评估 4D-MRI 到 CT 配准性能。方案适配进一步分析通过为 15 名寡转移性肝病患者制定合适的治疗计划，进行计算机模拟。结果。可以从 4D-MRI 扫描和 4D-MRI 中可靠地提取幻影轨迹，CT 配准显示亚毫米级精度。DAH 分析证明了对剂量评估结构 GTV 和 GTV _{TD 的}出色覆盖。患者图像中每日刚性 4D-MRI 至 midP-CT 配准精度的中位数<2 mm。ATP 策略在不增加暴露于 OAR 的情况下恢复了目标剂量，并且计划质量与 1-26 毫米范围内的 3D 移动距离无关。结论。提议的 4D-MRI 引导策略在所有工作流程测试中都表现出出色的性能，为 Unity MR-linac 的临床介绍做准备。