Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA.
Med Phys. 2011 Jul;38(7):4001-8. doi: 10.1118/1.3596776.
Hypofractionated prostate radiotherapy may benefit from both volumetric modulated are therapy (VMAT) due to shortened treatment time and intrafraction real-time monitoring provided by implanted radiofrequency(RF) transponders. The authors investigate dosimetrically driven action thresholds (whether treatment needs to be interrupted and patient repositioned) in VMAT treatment with electromagnetic (EM) tracking.
VMAT plans for five patients are generated for prescription doses of 32.5 and 42.5 Gy in five fractions. Planning target volume (PTV) encloses the clinical target volume (CTV) with a 3 mm margin at the prostate-rectal interface and 5 mm elsewhere. The VMAT delivery is modeled using 180 equi-spaced static beams. Intrafraction prostate motion is simulated in the plan by displacing the beam isocenter at each beam assuming rigid organ motion according to a previously recorded trajectory of the transponder centroid. The cumulative dose delivered in each fraction is summed over all beams. Two sets of 57 prostate motion trajectories were randomly selected to form a learning and a testing dataset. Dosimetric end points including CTV D95%, rectum wall D1cc, bladder wall D1cc, and urethra Dmax, are analyzed against motion characteristics including the maximum amplitude of the anterior-posterior (AP), superior-inferior (SI), and left-right components. Action thresholds are triggered when intrafraction motion causes any violations of dose constraints to target and organs at risk (OAR), so that treatment is interrupted and patient is repositioned.
Intrafraction motion has a little effect on CTV D95%, indicating PTV margins are adequate. Tight posterior and inferior action thresholds around 1 mm need to be set in a patient specific manner to spare organs at risk, especially when the prescription dose is 42.5 Gy. Advantages of setting patient specific action thresholds are to reduce false positive alarms by 25% when prescription dose is low, and increase the sensitivity of detecting dose limits violations by 30% when prescription dose is high, compared to a generic 2 mm action box. The sensitivity and specificity calculated from the testing dataset are consistent to the learning set, which indicates that the patient specific approach is reliable and reproducible within the scope of the prostate database.
This work introduces a formalism for ensuring a VMAT delivery meets the most clinically important dose requirements by using patient specific and dosimetric-driven action thresholds to hold the beam and reposition the patient when necessary. Such methods can provide improved sensitivity and specificity compared to conventional methods, which assume directionally symmetric action thresholds.
由于缩短了治疗时间和内置射频（RF）应答器提供的分次内实时监测，容积调强弧形治疗（VMAT）可能有益于前列腺的分段放射治疗。作者研究了在具有电磁（EM）跟踪的 VMAT 治疗中，剂量驱动的动作阈值（是否需要中断治疗并重新定位患者）。
为 5 名患者生成了处方剂量为 32.5 和 42.5 Gy 的 5 个分次的 VMAT 计划。计划靶区（PTV）在前列腺直肠界面处以 3mm 的边缘包含临床靶区（CTV），在其他部位以 5mm 的边缘包含。VMAT 输送通过 180 个等距静态射束建模。在计划中，通过根据先前记录的应答器质心轨迹在每个射束处移动射束等中心来模拟分次内前列腺运动。在每个分次中，将所有射束的累积剂量相加。随机选择两组 57 个前列腺运动轨迹，形成学习数据集和测试数据集。分析了包括 CTV D95%、直肠壁 D1cc、膀胱壁 D1cc 和尿道 Dmax 在内的剂量学终点，这些终点与前-后（AP）、上-下（SI）和左-右（LR）分量的最大幅度等运动特征有关。当分次内运动导致靶区和危及器官（OAR）的任何剂量限制违反时，触发动作阈值，从而中断治疗并重新定位患者。
分次内运动对 CTV D95%的影响很小，表明 PTV 边缘足够。需要针对每个患者设置严格的后向和下向动作阈值约 1mm，以保护危及器官，尤其是当处方剂量为 42.5 Gy 时。与通用的 2mm 动作框相比，设置患者特定动作阈值的优点是，当处方剂量较低时，将假阳性警报减少 25%，当处方剂量较高时，将检测到剂量限制违反的灵敏度提高 30%。从测试数据集计算出的敏感性和特异性与学习集一致，这表明在前列腺数据库范围内，患者特定方法是可靠且可重复的。
这项工作通过使用患者特定和剂量驱动的动作阈值来保持射束并在必要时重新定位患者，为确保 VMAT 输送满足最具临床意义的剂量要求引入了一种形式。与假设方向对称动作阈值的传统方法相比，这些方法可以提供更高的敏感性和特异性。
