Karagoz Gulay, Zorlu Faruk, Yeginer Mete, Yildiz Demet, Ozyigit Gokhan
Hacettepe University.
J Appl Clin Med Phys. 2016 Mar 8;17(2):14-23. doi: 10.1120/jacmp.v17i2.5474.
Accuracy and precision of leaf positioning in multileaf collimators (MLCs) are significant factors for the accuracy of IMRT treatments. This study aimed to inves-tigate the accuracy and repeatability of the MLC leaf positioning via the DAVID invivo dosimetric system for dynamic and static MLC systems. The DAVID system was designed as multiwire transmission ionization chamber which is placed in accessory holder of linear accelerators. Each wire of DAVID system corresponds to a MLC leaf-pair to verify the leaf positioning accuracy during IMRT treatment and QA. In this study, verifications of IMRT plans of five head and neck (H&N) and five prostate patients treated in a Varian DHX linear accelerator with 80-leaf MLC were performed using DAVID system. Before DAVID-based dosimetry, Electronics Portal Imaging Device (EPID) and PTW 2D ARRAY dosimetry system were used for 2D verification of each plan. The measurements taken by DAVID system in the first day of the treatments were used as reference for the following measurements taken over the next four weeks. The deviations in leaf positioning were evaluated by "Total Deviation (TD)" parameter calculated by DAVID software. The delivered IMRT plans were originally prepared using dynamic MLC method. The same plans were subsequently calculated based on static MLC method with three different intensity levels of five (IL5), 10 (IL10) and 20 (IL20) in order to compare the performances of MLC leaf positioning repeatability for dynamic and static IMRT plans. The leaf positioning accuracy is also evaluated by analyzing DynaLog files based on error histograms and root mean square (RMS) errors of leaf pairs' positions. Moreover, a correlation analysis between simultaneously taken DAVID and EPID measurements and DynaLog file recordings was subsequently performed. In the analysis of DAVID outputs, the overall deviations of dynamic MLC-based IMRT calculated from the deviations of the four weeks were found as 0.55% ± 0.57% and 1.48% ± 0.57% for prostate and H&N patients, respectively. The prostate IMRT plans based on static MLC method had the overall deviations of 1.23% ± 0.69%, 3.07% ± 1.07%, and 3.13% ± 1.29% for intensity levels of IL5, IL10, and IL20, respectively. Moreover, the overall deviations for H&N patients were found as 1.87% ± 0.86%, 3.11% ± 1.24%, and 2.78% ± 1.31% for the static MLC-based IMRT plans with intensity levels of IL5, IL10 and IL20, respectively. Similar with the DAVID results, the error rates in DynaLog files showed upward movement comparing the dynamic IMRT with static IMRT with high intensity levels. In respect to positioning errors higher than 0.005 cm, static prostate IMRT plans with intensity levels of IL10 and IL20 had 1.5 and 2.6 times higher error ratios than dynamic prostate IMRT plans, respectively, while these values stepped up to 8.4 and 12.0 for H&N cases. On the other hand, according to the leaf pair readings, reconstructed dose values from DynaLog files had significant correlation (r = 0.80) with DAVID and EPID readings while a stronger relationship (r = 0.98) was found between the two dosimetric systems. The correlation coefficients for deviations from reference plan readings were found in the interval of -0.21-0.16 for all three systems. The dynamic MLC method showed higher performance in repeatability of leaf positioning than static MLC methods with higher intensity levels even though the deviations in the MLC leaf positioning were found to be under the acceptance threshold for all MLC methods. The high intensity levels increased the position-ing deviations along with the delivery complexity of the static MLC-based IMRT plans. Moreover, DAVID and EPID readings and DynaLog recordings showed mutually strong correlation, while no significant relationship was found between deviations from reference values.
多叶准直器（MLC）中叶片定位的准确性和精确性是调强放射治疗（IMRT）准确性的重要因素。本研究旨在通过DAVID体内剂量测定系统研究动态和静态MLC系统中MLC叶片定位的准确性和可重复性。DAVID系统设计为多丝传输电离室，放置在线性加速器的附件支架中。DAVID系统的每根丝对应一对MLC叶片，以在IMRT治疗和质量保证期间验证叶片定位的准确性。在本研究中，使用DAVID系统对在配备80叶MLC的Varian DHX线性加速器中治疗的5例头颈部（H&N）患者和5例前列腺患者的IMRT计划进行了验证。在基于DAVID的剂量测定之前，使用电子射野影像装置（EPID）和PTW 2D ARRAY剂量测定系统对每个计划进行二维验证。将DAVID系统在治疗第一天的测量结果用作接下来四周内进行的后续测量的参考。叶片定位偏差通过DAVID软件计算的“总偏差（TD）”参数进行评估。最初使用动态MLC方法准备交付的IMRT计划。随后基于静态MLC方法，以5（IL5）、10（IL10）和20（IL20）三种不同强度水平计算相同的计划，以比较动态和静态IMRT计划中MLC叶片定位可重复性的性能。还通过基于误差直方图和叶片对位置的均方根（RMS）误差分析DynaLog文件来评估叶片定位准确性。此外，随后对同时进行的DAVID和EPID测量与DynaLog文件记录进行了相关性分析。在对DAVID输出的分析中，发现基于动态MLC的前列腺和H&N患者IMRT的四周偏差计算得出的总体偏差分别为0.55%±0.57%和1.48%±0.57%。基于静态MLC方法的前列腺IMRT计划在IL5、IL10和IL20强度水平下的总体偏差分别为1.23%±0.69%、3.07%±1.07%和3.13%±- 1.29%。此外，发现基于静态MLC的IMRT计划在IL5、IL10和IL20强度水平下，H&N患者的总体偏差分别为1.87%±0.86%、3.11%±1.24%和2.78%±1.31%。与DAVID结果类似，DynaLog文件中的错误率显示，与高强度水平的静态IMRT相比，动态IMRT呈上升趋势。对于高于0.005 cm的定位误差，强度水平为IL10和IL20的静态前列腺IMRT计划的错误率分别比动态前列腺IMRT计划高1.5倍和2.6倍，而对于H&N病例，这些值分别升至8.4倍和12.0倍。另一方面，根据叶片对读数，DynaLog文件重建的剂量值与DAVID和EPID读数具有显著相关性（r = 0.80），而在两个剂量测定系统之间发现了更强的关系（r = 0.98）。所有三个系统与参考计划读数偏差的相关系数在- / 0.21 - 0.16范围内。动态MLC方法在叶片定位可重复性方面表现出比高强度水平的静态MLC方法更高的性能，尽管发现所有MLC方法的MLC叶片定位偏差均在可接受阈值以下。高强度水平增加了基于静态MLC的IMRT计划的定位偏差以及交付复杂性。此外，DAVID和EPID读数与DynaLog记录显示出相互强烈的相关性，而与参考值的偏差之间未发现显著关系。
