Department of Radiation Oncology, University of Pittsburgh Cancer Institute, Pittsburgh, PA 15232, USA.
Phys Med Biol. 2011 Mar 7;56(5):1385-95. doi: 10.1088/0031-9155/56/5/011. Epub 2011 Feb 11.
An external surrogate-based respiratory gating technique is a useful method to reduce target margins for the treatment of a moving lung tumor. The success of this technique relies on a good correlation between the motion of the external markers and the internal tumor as well as the repeatability of the respiratory motion. In gated lung stereotactic body radiation therapy (SBRT), the treatment time for each fraction could exceed 30 min due to large fractional dose. Tumor motion may experience pattern changes such as baseline shift during such extended treatment time. The purpose of this study is to analyze tumor motion traces in actual treatment situations and to evaluate the effect of the target baseline shift in gated lung SBRT treatment. Real-time motion data for both the external markers and tumors from 51 lung SBRT treatments with Cyberknife Synchrony technology were analyzed in this study. The treatment time is typically greater than 30 min. The baseline shift was calculated with a rolling average window equivalent to ∼20 s and subtracted from that at the beginning. The magnitude of the baseline shift and its relationship with treatment time were investigated. Phase gating simulation was retrospectively performed on 12 carefully selected treatments with respiratory amplitude larger than 5 mm and regular phases. A customized gating window was defined for each individual treatment. It was found that the baseline shifts are specific to each patient and each fraction. Statistical analysis revealed that more than 69% treatments exhibited increased baseline shifts with the lapse of treatment time. The magnitude of the baseline shift could reach 5.3 mm during a 30 min treatment. Gating simulation showed that tumor excursion was caused mainly by the uncertainties in phase gating simulation and baseline shift, the latter being the primary factor. With a 5 mm gating window, 2 out of 12 treatments in the study group showed significant tumor excursion. Baseline shifts alone could cause up to 20% of tumor excursion outside the gating window. It is concluded that baseline shifts may increase with the treatment time and are more likely to act as a time-dependent systematic error. For phase-based gated lung SBRT, a baseline shift may be one of the major sources of targeting error during treatment.
基于外部替代物的呼吸门控技术是一种减少移动性肺肿瘤靶区边缘的有效方法。该技术的成功依赖于外部标记物与内部肿瘤运动之间的良好相关性以及呼吸运动的可重复性。在门控肺部立体定向体部放射治疗（SBRT）中，由于分次剂量较大，每个分次的治疗时间可能超过 30 分钟。在如此长的治疗时间内，肿瘤运动可能会经历基线偏移等模式变化。本研究的目的是分析实际治疗情况下的肿瘤运动轨迹，并评估门控肺部 SBRT 治疗中靶区基线偏移的影响。本研究分析了使用 Cyberknife Synchrony 技术的 51 例肺部 SBRT 治疗的实时外部标记物和肿瘤运动数据。治疗时间通常大于 30 分钟。使用与约 20 秒等效的滚动平均窗口计算基线偏移，并从开始时减去该值。研究了基线偏移的幅度及其与治疗时间的关系。对 12 例呼吸幅度大于 5 毫米且相位规则的精选治疗进行了回顾性相位门控模拟。为每个单独的治疗定义了一个定制的门控窗口。结果发现，基线偏移是每个患者和每个分次特有的。统计分析显示，超过 69%的治疗随着治疗时间的推移显示出基线偏移增加。在 30 分钟的治疗中，基线偏移幅度可达 5.3 毫米。门控模拟显示，肿瘤位移主要是由相位门控模拟和基线偏移的不确定性引起的，后者是主要因素。在 5 毫米的门控窗口下，研究组的 12 例治疗中有 2 例显示出明显的肿瘤位移。仅基线偏移就可能导致高达 20%的肿瘤超出门控窗口。综上所述，基线偏移可能会随治疗时间而增加，更有可能成为一种时间相关的系统误差。对于基于相位的门控肺部 SBRT，基线偏移可能是治疗过程中靶区定位误差的主要来源之一。
