Use of EPID for leaf position accuracy QA of dynamic multi-leaf collimator (DMLC) treatment
Use of EPID for leaf position accuracy QA of dynamic multi-leaf collimator (DMLC) treatment
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DOI：
10.1118/1.1760187
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发表时间：
2004-07-01
期刊：
影响因子：
3.8
通讯作者：
Burman, C
中科院分区：
文献类型：
--
作者：
Chang, J;Obcemea, CH;Burman, C
本文描述了一种使用电子射野成像设备（EPID）进行常规动态多叶准直器（DMLC）质量保证（QA）的替代方法。目前，该QA在我们的机构通过拍摄强度调制放射治疗（IMRT）测试场来完成，该测试场产生5个间隔2 cm的1 mm带的图案，并对叶对齐、运动滞后、粘连和任何其他机械问题进行目视抽查。在这项研究中，我们使用非晶硅aS 500 EPID和薄膜同时用于DMLC QA，以测试EPID的实用性和有效性。通过首先使用校准曲线将阅读转换为剂量，然后乘以平均帧数，将EPID图像转换为积分剂量图。然后将EPID剂量图反向投影到中心轴平面，并与胶片测量值进行比较，使用胶片剂量测定系统扫描并转换为剂量。我们确定了两张图像每个谱带的半高全宽（FWHM），并评估了两个峰之间谷的剂量。我们还模拟机械问题，通过增加带隙到1.5毫米的一些叶片对。我们的结果表明，EPID是一样好的解决调强放射治疗测试场的频带模式的电影。虽然EPID的分辨率低于胶片的分辨率（0.78 mm/像素对胶片的0.36 min/像素），但它足够高，可以忠实地再现条带图案，而不会出现明显的失真。EPID的FWHM为2.84 mm，略高于膜的2.01 mm。EPID（峰值的15.5%）的最低谷剂量显著低于胶片（28.6%），表明EPID的能量独立性较低。模拟的叶子问题可以通过两个图像的视觉检查发现;然而，对于没有扫描和对比度增强的胶片来说，这是更困难的。EPID图像的优点是已经是数字化的，并且它们的分析可以容易地自动化，以标记超出设定参数（例如FWHM、峰值剂量值、峰值位置和峰值之间的距离）的公差限制的叶对。这种自动化是一种新功能，将有助于在实际IMRT治疗期间抢占MLC运动联锁并减少机器停机时间。我们的结论是，由于EPID图像可以更方便地获取，分析和存储比电影，EPID是一个很好的替代电影常规DMLC QA。(C)2004年美国医学物理学家协会。
We describe in this paper an alternative method for routine dynamic multi-leaf collimator (DMLC) quality assurance (QA) using an electronic portal imaging device (EPID). Currently, this QA is done at our institution by filming an intensity-modulated radiotherapy (IMRT) test field producing a pattern of five 1-mm bands 2 cm apart and performing a visual spot-check for leaf alignment, motion lags, sticking and any other mechanical problems. In this study, we used an amorphous silicon aS500 EPID and films contemporaneously for the DMLC QA to test the practicality and efficacy of EPID vis-a-vis film. The EPID image was transformed to an integrated dose map by first converting the reading to dose using a calibration curve, and then multiplying by the number of averaged frames. The EPID dose map was then back-projected to the central axis plane and was compared to the film measurements which were scanned and converted to dose using a film dosimetry system. We determined the full-width half-maximum (FWHM) of each band for both images, and evaluated the dose to the valley between two peaks. We also simulated mechanical problems by increasing the band gap to 1.5 mm for some leaf pairs. Our results show that EPID is as good as the film in resolving the band pattern of the IMRT test field. Although the resolution of the EPID is lower than that of the film (0.78 mm/pixel vs 0.36 min/pixel for the film), it is high enough to faithfully reproduce the band pattern without significant distortion. The FWHM of the EPID is 2.84 mm, slightly higher than the 2.01 mm for the film. The lowest dose to the valley is significantly lower for the EPID (15.5% of the peak value) than for the film (28.6%), indicating that EPID is less energy independent. The simulated leaf problem can be spotted by visual inspection of both images; however, it is more difficult for the film without being scanned and contrast-enhanced. EPID images have the advantage of being already digital and their analysis can easily be automated to flag leaf pairs outside tolerance limits of set parameters such as FWHM, peak dose values, peak location, and distance between peaks. This automation is a new feature that will help preempt MLC motion interlocks and decrease machine downtime during actual IMRT treatment. We conclude that since EPID images can be acquired, analyzed and stored much more conveniently than film, EPID is a good alternative to film for routine DMLC QA. (C) 2004 American Association of Physicists in Medicine.