Medical Physics Laboratory, Medical School, National and Kapodistrian University of Athens, Athens, Greece.
Medical Physics Department, Hygeia Hospital, Athens, Greece.
J Appl Clin Med Phys. 2022 Aug;23(8):e13708. doi: 10.1002/acm2.13708. Epub 2022 Jun 22.
To study the impact of systematic MLC leaf positional uncertainties (stemming from mechanical inaccuracies or sub-optimal MLC modeling) on the quality of intracranial single-isocenter multi-target VMAT-SRS treatment plans. An estimation of appropriate tolerance levels is attempted.
Five patients, with three to four metastases and at least one target lying in close proximity to organs-at-risk (OARs) were included in this study. A single-isocenter multi-arc VMAT plan per patient was prepared, which served as the reference for dosimetric impact evaluation. A range of leaf offsets was introduced (±0.03 mm up to ±0.30 mm defined at the MLC plane) to both leaf banks, by varying the leaf offset MLC modeling parameter in Monaco for all the prepared plans, in order to simulate projected leaf offsets of ±0.09 mm up to ±0.94 mm at the isocenter plane, respectively. For all offsets simulated and cases studied, dose distributions were re-calculated and compared with the corresponding reference ones. An experimental dosimetric procedure using the SRS mapCHECK diode array was also performed to support the simulation study results and investigate its suitability to detect small systematic leaf positional errors.
Projected leaf offsets of ±0.09 mm were well-tolerated with respect to both target dosimetry and OAR-sparing. A linear relationship was found between D percentage change and projected leaf offset (slope: 12%/mm). Impact of projected offset on target dosimetry was strongly associated with target volume. In two cases, plans that could be considered potentially clinically unacceptable (i.e., clinical dose constraint violation) were obtained even for projected offsets as small as 0.19 mm. The performed experimental dosimetry check can detect potential small systematic leaf errors.
Plan quality indices and dose-volume metrics are very sensitive to systematic sub-millimeter leaf positional inaccuracies, projected at the isocenter plane. Acceptable and tolerance levels in systematic MLC uncertainties need to be tailored to VMAT-SRS spatial and dosimetric accuracy requirements.
研究系统多叶准直器（MLC）叶片位置不确定性（源于机械误差或 MLC 建模不当）对颅内单中心多靶容积旋转调强弧形放射治疗（VMAT-SRS）计划质量的影响。尝试对合适的容限水平进行估计。
本研究纳入了 5 名患者，每位患者均有 3 至 4 个转移病灶，至少有一个病灶紧邻危及器官（OAR）。每位患者均准备了一个单中心多弧 VMAT 计划作为剂量学影响评估的参考。通过改变 Monaco 计划系统中所有准备好的计划的叶片偏移 MLC 建模参数，在两个叶片组上分别引入±0.03mm 至±0.30mm 的叶片偏移范围，以模拟在等中心平面上分别为±0.09mm 至±0.94mm 的预测叶片偏移。对于所有模拟的偏移量和研究的病例，重新计算剂量分布并与相应的参考剂量分布进行比较。还使用 SRS mapCHECK 二极管阵列进行了实验剂量学检查，以支持模拟研究结果并研究其检测小的系统叶片位置误差的适用性。
对于靶区剂量和 OAR 保护，±0.09mm 的预测叶片偏移量是可以接受的。发现 D 百分比变化与预测叶片偏移量之间存在线性关系（斜率：12%/mm）。目标剂量学受预测偏移量的影响与目标体积密切相关。在两个病例中，即使对于预测偏移量小至 0.19mm，也获得了可能被认为是潜在临床不可接受的计划（即临床剂量限制约束违反）。所进行的实验剂量学检查可以检测到潜在的小系统叶片误差。
在等中心平面上预测的系统亚毫米叶片位置不准确性对计划质量指数和剂量-体积指标非常敏感。在系统 MLC 不确定性方面，需要根据 VMAT-SRS 的空间和剂量学精度要求来确定可接受的和容忍的容限水平。
