Dvorak Pavel, Georg Dietmar, Bogner Joachim, Kroupa Bernhard, Dieckmann Karin, Pötter Richard
Department of Radiotherapy and Radiobiology, AKH Vienna, Medical University of Vienna, Vienna, Austria.
Int J Radiat Oncol Biol Phys. 2005 Apr 1;61(5):1572-81. doi: 10.1016/j.ijrobp.2004.12.075.
The present study explored the impact of intensity-modulated radiotherapy (IMRT) on stereotactic body RT (SBRT) of liver and lung lesions. Additionally, because target dose conformity can be affected by the leaf width of a multileaf collimator (MLC), especially for small targets and stereotactic applications, the use of a micro-MLC on "uniform intensity" conformal and intensity-modulated SBRT was evaluated.
The present study included 10 patients treated previously with SBRT in our institution (seven lung and three liver lesions). All patients were treated with 3 x 12 Gy prescribed to the 65% isodose level. The actual MLC-based conformal treatment plan served as the standard for additional comparison. In total, seven alternative treatment plans were made for each patient: a standard (actual) plan and an IMRT plan, both calculated with Helax TMS (Nucletron) using a pencil beam model; and a recalculated standard and a recalculated IMRT plan on Helax TMS using a point dose kernel approach. These four treatment plans were based on a standard MLC with 1-cm leaf width. Additionally, the following micro-MLC (central leaf width 3 mm)-based treatment plans were calculated with the BrainSCAN (BrainLAB) system: standard, IMRT, and dynamic arc treatments. For each treatment plan, various target parameters (conformity, coverage, mean, maximal, and minimal target dose, equivalent uniform doses, and dose-volume histogram), as well as organs at risk parameters (3 Gy and 6 Gy volume, mean dose, dose-volume histogram) were evaluated. Finally, treatment efficiency was estimated from monitor units and the number of segments for IMRT solutions.
For both treatment planning systems, no significant difference could be observed in terms of target conformity between the standard and IMRT dose distributions. All dose distributions obtained with the micro-MLC showed significantly better conformity values compared with the standard and IMRT plans using a regular MLC. Dynamic arc plans were characterized by the steepest dose gradient and thus the smallest V(6 Gy) values, which were on average 7% smaller than the standard plans and 20% lower than the IMRT plans. Although the Helax TMS IMRT plans show about 18% more monitor units than the standard plan, BrainSCAN IMRT plans require approximately twice the number of monitor units relative to the standard plan. All treatment plans optimized with a pencil beam model but recalculated with a superposition method showed significant qualitative, as well as quantitative, differences, especially with respect to conformity and the dose to organs at risk.
Standard conformal treatment techniques for SBRT could not be improved with inversely planned IMRT approaches. Dose calculation algorithms applied in optimization modules for IMRT applications in the thoracic region need to be based on the most accurate dose calculation algorithms, especially when using higher energy photon beams.
本研究探讨调强放疗（IMRT）对肝脏和肺部病变立体定向体部放疗（SBRT）的影响。此外，由于多叶准直器（MLC）的叶片宽度可能会影响靶区剂量适形度，特别是对于小靶区和立体定向应用，因此评估了在“均匀强度”适形和调强SBRT中使用微型MLC的情况。
本研究纳入了10例先前在我院接受SBRT治疗的患者（7例肺部病变和3例肝脏病变）。所有患者均接受3×12 Gy的剂量，处方剂量至65%等剂量线。基于实际MLC的适形治疗计划作为额外比较的标准。总共为每位患者制定了7种替代治疗计划：一个标准（实际）计划和一个IMRT计划，两者均使用铅笔束模型在Helax TMS（Nucletron）上计算；以及在Helax TMS上使用点剂量核方法重新计算的标准计划和重新计算的IMRT计划。这四种治疗计划基于叶片宽度为1 cm的标准MLC。此外，使用BrainSCAN（BrainLAB）系统计算了以下基于微型MLC（中心叶片宽度3 mm）的治疗计划：标准、IMRT和动态弧形治疗。对于每个治疗计划，评估了各种靶区参数（适形度、覆盖度、平均、最大和最小靶区剂量、等效均匀剂量和剂量体积直方图）以及危及器官参数（3 Gy和6 Gy体积、平均剂量、剂量体积直方图）。最后，根据IMRT方案的监测单位和射野段数估计治疗效率。
对于两个治疗计划系统，标准剂量分布和IMRT剂量分布在靶区适形度方面未观察到显著差异。与使用常规MLC的标准计划和IMRT计划相比，所有使用微型MLC获得的剂量分布显示出明显更好的适形度值。动态弧形计划的特点是剂量梯度最陡，因此V（6 Gy）值最小，平均比标准计划小7%，比IMRT计划低20%。尽管Helax TMS IMRT计划的监测单位比标准计划多约18%，但BrainSCAN IMRT计划相对于标准计划需要的监测单位数量约为两倍。所有用铅笔束模型优化但用叠加方法重新计算的治疗计划在质量和数量上均显示出显著差异，特别是在适形度和危及器官的剂量方面。
SBRT的标准适形治疗技术无法通过逆向计划的IMRT方法得到改善。用于胸部IMRT应用优化模块的剂量计算算法需要基于最准确的剂量计算算法，特别是在使用更高能量光子束时。
