Vieira S C, Dirkx M L P, Pasma K L, Heijmen B J M
Department of Radiotherapy, Division of Clinical Physics, Erasmus MC/Daniel den Hoed Cancer Center, Groene Hilledijk 301, 3075 EA Rotterdam, The Netherlands.
Phys Med Biol. 2003 Jan 21;48(2):157-66. doi: 10.1088/0031-9155/48/2/302.
Regions with steep dose gradients are often encountered in clinical x-ray beams, especially with the growing use of intensity modulated radiotherapy (IMRT). Such regions are present both at field edges and, for IMRT, in the vicinity of the projection of sensitive anatomical structures in the treatment field. Dose measurements in these regions are often difficult and labour intensive, while dose prediction may be inaccurate. A dedicated algorithm developed in our institution for conversion of pixel values, measured with a charged coupled device camera based fluoroscopic electronic portal imaging device (EPID), into absolute absorbed doses at the EPID plane has an accuracy of 1-2% for flat and smoothly modulated fields. However, in the current algorithm there is no mechanism to correct for the (short-range) differences in lateral electron transport between water and the metal plate with the fluorescent layer in the EPID. Moreover, lateral optical photon transport in the fluorescent layer is not taken into account. This results in large deviations (>10%) in the penumbra region of these fields. We have investigated the differences between dose profiles measured in water and with the EPID for small heavily peaked fields. A convolution kernel has been developed to empirically describe these differences. After applying the derived kernel to raw EPID images, a general agreement within 2% was obtained with the water measurements in the central region of the fields, and within 0.03 cm in the penumbra region. These results indicate that the EPID is well suited for accurate dosimetric verification of steep gradient x-ray fields.
在临床X射线束中经常会遇到剂量梯度陡峭的区域，尤其是随着调强放射治疗（IMRT）的日益广泛应用。这样的区域既存在于射野边缘，对于IMRT而言，也存在于治疗射野中敏感解剖结构投影的附近。在这些区域进行剂量测量往往困难且耗费人力，而剂量预测可能不准确。我们机构开发的一种专用算法，可将基于电荷耦合器件相机的荧光透视电子射野影像装置（EPID）所测量的像素值转换为EPID平面处的绝对吸收剂量，对于平坦且调制平滑的射野，其精度为1% - 2%。然而，在当前算法中，没有机制来校正水与EPID中带有荧光层的金属板之间横向电子输运的（短程）差异。此外，荧光层中的横向光学光子输运也未被考虑在内。这导致在这些射野的半影区域出现较大偏差（>10%）。我们研究了在水中和使用EPID测量的小的高剂量峰值射野的剂量分布差异。已开发出一种卷积核来经验性地描述这些差异。将导出的核应用于原始EPID图像后，在射野中心区域与水中测量结果的总体一致性在2%以内，在半影区域一致性在0.03 cm以内。这些结果表明，EPID非常适合对陡峭梯度X射线射野进行准确的剂量学验证。
