Abstract Purpose Patients undergoing 3DCRT on TrueBeam linac equipped with HD_120_MLC are subjected to localized extra-focal dose (LEFD) in a specific region out of the treatment field. The dose in this region is up to 160% of the mean transmitted dose through MLC. Although VMAT provides a more conformal dose to PTV and better sparing to OAR, it implies more monitor units and irradiating more healthy tissues outside the treatment field. One aims to investigate the dose distribution due to LEFD during VMAT and the collimator rotation angle optimization to minimize this extra dose. Methods Eight VMAT plans for a hypothetical abdominal tumor were contoured on an anthropomorphic pediatric phantom and calculated on Eclipse TPS. The plans were optimized to the same dosimetric objectives at PTV and OAR with different collimator rotation angles. They were also simulated using PRIMO Monte Carlo software. The dose out of the treatment field was evaluated in eight spherical structures distributed at different distances from the isocenter. To validate the results, QA verification plans were created, were measured by EPID and simulated in a virtual phantom of the EPID. Measured and simulated QA plans were compared using gamma-index methodology. Point dose measurements were also used to verify Monte Carlo simulations accuracy out of the treatment field. Results Both measured and simulated QA dose distributions showed that each treatment arc has two LEFD regions out of the treatment field which were not calculated by TPS. The simulated treatment plans showed LEFD as cylindrical dose distributions, 10 cm height, with the diameter of X-jaws aperture. At 0° rotation the two LEFD regions corresponding to the two-treatment arcs are superimposed. As rotation angle increases the two cylindrical distributions start to drift apart but their intersection remains localized along the central axis until they are completely separated. Conclusion The LEFD can be avoided during VMAT without compromising the treatment plan objectives by increasing the collimator rotation angle to relocate LEFD out of the body.

中文翻译：

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VMAT 期间局部额外焦点剂量准直器角度依赖性：使用 PRIMO 软件的场外蒙特卡罗研究

摘要 目的在配备 HD_120_MLC 的 TrueBeam 直线加速器上接受 3DCRT 的患者在治疗区域外的特定区域接受局部焦外剂量 (LEFD)。该区域的剂量高达通过 MLC 的平均透射剂量的 160%。尽管 VMAT 为 PTV 提供了更适形的剂量并更好地保留了 OAR，但它意味着更多的监测单元和照射治疗场外的更多健康组织。一个目的是研究 VMAT 和准直器旋转角优化期间由于 LEFD 引起的剂量分布，以尽量减少这种额外剂量。方法 8 个用于假设腹部肿瘤的 VMAT 计划在拟人化的儿科模型上绘制轮廓，并在 Eclipse TPS 上计算。这些计划针对具有不同准直器旋转角度的 PTV 和 OAR 的相同剂量测定目标进行了优化。他们还使用 PRIMO Monte Carlo 软件进行了模拟。在分布在距等中心点不同距离的八个球形结构中评估了治疗野的剂量。为了验证结果，创建了 QA 验证计划，由 EPID 测量并在 EPID 的虚拟模型中进行模拟。使用伽马指数方法比较测量和模拟的质量保证计划。点剂量测量也用于验证治疗场外的蒙特卡罗模拟准确性。结果 测量的和模拟的 QA 剂量分布都表明，每个治疗弧在治疗场外都有两个 LEFD 区域，这些区域不是由 TPS 计算的。模拟治疗计划将 LEFD 显示为圆柱形剂量分布，高度为 10 厘米，直径为 X 颌孔。在 0° 旋转时，对应于两个治疗弧的两个 LEFD 区域叠加。随着旋转角度的增加，两个圆柱分布开始逐渐分开，但它们的交点仍然沿中心轴局部化，直到它们完全分开。结论 通过增加准直器旋转角度将 LEFD 移出体外，可以在 VMAT 期间避免 LEFD，而不会影响治疗计划目标。