Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
Med Phys. 2012 Sep;39(9):5557-66. doi: 10.1118/1.4745562.
Dose-rate-regulated tracking (DRRT) is a tumor tracking strategy that programs the MLC to track the tumor under regular breathing and adapts to breathing irregularities during delivery using dose rate regulation. Constant-dose-rate tracking (CDRT) is a strategy that dynamically repositions the beam to account for intrafractional 3D target motion according to real-time information of target location obtained from an independent position monitoring system. The purpose of this study is to illustrate the differences in the effectiveness and delivery accuracy between these two tracking methods in the presence of breathing irregularities.
Step-and-shoot IMRT plans optimized at a reference phase were extended to remaining phases to generate 10-phased 4D-IMRT plans using segment aperture morphing (SAM) algorithm, where both tumor displacement and deformation were considered. A SAM-based 4D plan has been demonstrated to provide better plan quality than plans not considering target deformation. However, delivering such a plan requires preprogramming of the MLC aperture sequence. Deliveries of the 4D plans using DRRT and CDRT tracking approaches were simulated assuming the breathing period is either shorter or longer than the planning day, for 4 IMRT cases: two lung and two pancreatic cases with maximum GTV centroid motion greater than 1 cm were selected. In DRRT, dose rate was regulated to speed up or slow down delivery as needed such that each planned segment is delivered at the planned breathing phase. In CDRT, MLC is separately controlled to follow the tumor motion, but dose rate was kept constant. In addition to breathing period change, effect of breathing amplitude variation on target and critical tissue dose distribution is also evaluated.
Delivery of preprogrammed 4D plans by the CDRT method resulted in an average of 5% increase in target dose and noticeable increase in organs at risk (OAR) dose when patient breathing is either 10% faster or slower than the planning day. In contrast, DRRT method showed less than 1% reduction in target dose and no noticeable change in OAR dose under the same breathing period irregularities. When ±20% variation of target motion amplitude was present as breathing irregularity, the two delivery methods show compatible plan quality if the dose distribution of CDRT delivery is renormalized.
Delivery of 4D-IMRT treatment plans, stemmed from 3D step-and-shoot IMRT and preprogrammed using SAM algorithm, is simulated for two dynamic MLC-based real-time tumor tracking strategies: with and without dose-rate regulation. Comparison of cumulative dose distribution indicates that the preprogrammed 4D plan is more accurately and efficiently conformed using the DRRT strategy, as it compensates the interplay between patient breathing irregularity and tracking delivery without compromising the segment-weight modulation.
剂量率调节跟踪（DRRT）是一种肿瘤跟踪策略，它通过剂量率调节使 MLC 在常规呼吸下跟踪肿瘤，并在输送过程中适应呼吸不规则。恒剂量率跟踪（CDRT）是一种策略，根据从独立位置监测系统获得的目标位置实时信息，动态重新定位光束以补偿分次内 3D 靶区运动。本研究的目的是说明在呼吸不规则存在的情况下，这两种跟踪方法在有效性和输送精度方面的差异。
使用节段孔径变形（SAM）算法将在参考相位优化的步进和射击 IMRT 计划扩展到剩余相位，以生成 10 相位 4D-IMRT 计划，其中同时考虑了肿瘤位移和变形。基于 SAM 的 4D 计划已被证明比不考虑目标变形的计划提供更好的计划质量。然而，输送这样的计划需要预先编程 MLC 孔径序列。对于 4 个 IMRT 病例（两个肺和两个胰腺病例，最大 GTV 质心运动大于 1cm），假设呼吸周期短于或长于计划日，模拟了使用 DRRT 和 CDRT 跟踪方法输送 4D 计划。在 DRRT 中，根据需要调节剂量率以加快或减慢输送速度，以便按计划的呼吸相位输送每个计划的节段。在 CDRT 中，单独控制 MLC 以跟随肿瘤运动，但保持剂量率不变。除了呼吸周期变化之外，还评估了呼吸幅度变化对靶区和关键组织剂量分布的影响。
使用 CDRT 方法输送预编程的 4D 计划会导致靶区剂量平均增加 5%，并且当患者的呼吸比计划日快 10%或慢 10%时，危及器官（OAR）剂量明显增加。相比之下，在相同的呼吸周期不规则下，DRRT 方法导致靶区剂量减少不到 1%，OAR 剂量没有明显变化。当目标运动幅度的变化为呼吸不规则的±20%时，如果 CDRT 输送的剂量分布重新归一化，则两种输送方法的计划质量具有可比性。
使用基于 3D 步进和射击 IMRT 的 4D-IMRT 治疗计划，并使用 SAM 算法进行预编程，针对两种基于动态 MLC 的实时肿瘤跟踪策略进行模拟：有和没有剂量率调节。累积剂量分布的比较表明，使用 DRRT 策略更准确、更有效地符合预编程的 4D 计划，因为它补偿了患者呼吸不规则和跟踪输送之间的相互作用，而不会影响节段权重调制。
