Department of Radiation Oncology, The Netherlands Cancer Institute--Antoni van Leeuwenhoek Hospital, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands.
Med Phys. 2012 Jan;39(1):367-77. doi: 10.1118/1.3665709.
At the Netherlands Cancer Institute--Antoni van Leeuwenhoek Hospital in vivo dosimetry using an electronic portal imaging device (EPID) has been implemented for almost all high-energy photon treatments of cancer with curative intent. Lung cancer treatments were initially excluded, because the original back-projection dose-reconstruction algorithm uses water-based scatter-correction kernels and therefore does not account for tissue inhomogeneities accurately. The aim of this study was to test a new method, in aqua vivo EPID dosimetry, for fast dose verification of lung cancer irradiations during actual patient treatment.
The key feature of our method is the dose reconstruction in the patient from EPID images, obtained during the actual treatment, whereby the images have been converted to a situation as if the patient consisted entirely of water; hence, the method is termed in aqua vivo. This is done by multiplying the measured in vivo EPID image with the ratio of two digitally reconstructed transmission images for the unit-density and inhomogeneous tissue situation. For dose verification, a comparison is made with the calculated dose distribution with the inhomogeneity correction switched off. IMRT treatment verification is performed for each beam in 2D using a 2D γ evaluation, while for the verification of volumetric-modulated arc therapy (VMAT) treatments in 3D a 3D γ evaluation is applied using the same parameters (3%, 3 mm). The method was tested using two inhomogeneous phantoms simulating a tumor in lung and measuring its sensitivity for patient positioning errors. Subsequently five IMRT and five VMAT clinical lung cancer treatments were investigated, using both the conventional back-projection algorithm and the in aqua vivo method. The verification results of the in aqua vivo method were statistically analyzed for 751 lung cancer patients treated with IMRT and 50 lung cancer patients treated with VMAT.
The improvements by applying the in aqua vivo approach are considerable. The percentage of γ values ≤1 increased on average from 66.2% to 93.1% and from 43.6% to 97.5% for the IMRT and VMAT cases, respectively. The corresponding mean γ value decreased from 0.99 to 0.43 for the IMRT cases and from 1.71 to 0.40 for the VMAT cases, which is similar to the accepted clinical values for the verification of IMRT treatments of prostate, rectum, and head-and-neck cancers. The deviation between the reconstructed and planned dose at the isocenter diminished on average from 5.3% to 0.5% for the VMAT patients and was almost the same, within 1%, for the IMRT cases. The in aqua vivo verification results for IMRT and VMAT treatments of a large group of patients had a mean γ of approximately 0.5, a percentage of γ values ≤1 larger than 89%, and a difference of the isocenter dose value less than 1%.
With the in aqua vivo approach for the verification of lung cancer treatments (IMRT and VMAT), we can achieve results with the same accuracy as obtained during in vivo EPID dosimetry of sites without large inhomogeneities.
在荷兰癌症研究所-安东尼范列文虎克医院，使用电子门户成像设备（EPID）进行体内剂量学已几乎应用于所有具有治愈意图的高能光子癌症治疗。最初排除了肺癌治疗，因为原始的反向投影剂量重建算法使用基于水的散射校正核，因此不能准确地考虑组织不均匀性。本研究的目的是测试一种新方法，即水活体 EPID 剂量学，用于在实际患者治疗过程中快速验证肺癌放疗的剂量。
我们方法的关键特征是从 EPID 图像中重建患者的剂量，这些图像是在实际治疗过程中获得的，其中图像已转换为患者完全由水组成的情况；因此，该方法称为水活体。这是通过将测量的活体 EPID 图像与单位密度和不均匀组织情况下的两个数字重建透射图像的比值相乘来实现的。为了进行剂量验证，将与未开启不均匀校正的计算剂量分布进行比较。对于每个光束，使用二维γ评估进行 IMRT 治疗验证，而对于三维调强弧形治疗（VMAT）治疗的验证，使用相同的参数（3％，3mm）进行三维γ评估。该方法使用模拟肺部肿瘤的两个不均匀体模进行了测试，并测量了其对患者定位误差的敏感性。随后，对 5 例 IMRT 和 5 例 VMAT 临床肺癌治疗进行了研究，同时使用传统的反向投影算法和水活体方法。对 751 例接受 IMRT 治疗的肺癌患者和 50 例接受 VMAT 治疗的肺癌患者的水活体方法的验证结果进行了统计学分析。
应用水活体方法的改进是相当可观的。对于 IMRT 和 VMAT 病例，γ值≤1的百分比平均从 66.2％增加到 93.1％和从 43.6％增加到 97.5％。对于 IMRT 病例，相应的平均γ值从 0.99 降低到 0.43，对于 VMAT 病例，从 1.71 降低到 0.40，这与前列腺、直肠和头颈部癌症的 IMRT 治疗验证的接受临床值相似。VMAT 患者的等中心处重建剂量与计划剂量之间的偏差平均从 5.3％降低到 0.5％，而 IMRT 病例的偏差几乎相同，在 1％以内。对于接受大量患者的 IMRT 和 VMAT 治疗的水活体验证结果，γ值的平均值约为 0.5，γ 值≤1的百分比大于 89％，等中心剂量值的差异小于 1％。
对于肺癌治疗（IMRT 和 VMAT）的水活体验证方法，我们可以获得与没有大的不均匀性的部位的体内 EPID 剂量学相同的准确性结果。
