An experimental investigation into the radiation field offset of a dynamic multileaf collimator
An experimental investigation into the radiation field offset of a dynamic multileaf collimator
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DOI：
10.1088/0031-9155/51/21/009
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发表时间：
2006-11-07
影响因子：
3.5
通讯作者：
Baldock, Clive
中科院分区：
文献类型：
--
作者：
Vial, Philip;Oliver, Lyn;Baldock, Clive
在这项研究中，我们研究了一种圆形叶端多叶准直器(MLC)的特性，该准直器用于使用VARIAN直线加速器进行调强放射治疗(IMRT)。圆形叶端MLC设计导致辐射场边缘(物理叶位置)和光场(几何叶位置)之间的偏移量。我们称之为辐射场偏移量(RFO)。将叶位置校准到叶中部平面上的叶尖。几何叶片位置和投影叶尖位置之间有一个额外的偏移量，该偏移量随着与准直器中心轴的距离而变化，这是由于MLC几何结构所致。我们称之为叶位置偏移量(LPO)。在文献中，对RFO和LPO的解释和执行缺乏一致性。我们研究了瓦里安的600C/D和21个直线加速器上的RFO和LPO。我们使用静态、分段MLC(SMLC)和动态MLC(DMLC)场的薄膜测量和电离室测量相结合的方法来量化叶片位置范围内的叶片偏移量。我们能够通过对供应商的LPO文件进行微小的调整来改进大的离轴位置的剂量测定。在准直器中心轴线上，RFO的测量精度在0.1 mm以内。测量的RFO值取决于该方法是基于辐射场边缘位置还是基于积分剂量测量。积分剂量法产生的RFO大约比辐射场边缘法大0.2 mm。这种差异是由于MLC的半影形状造成的。我们提出了一种同时适用于SMLC和DMLC调强RT的MLC叶偏移量的测量和实现方法。此外，我们还提出了一些更清楚地描述MLC叶位置的定义，以便进行精确的调强放射治疗剂量测定。
In this study we investigate the characteristics of a rounded leaf end multileaf collimator (MLC) that is used for delivering intensity-modulated radiotherapy (IMRT) with a Varian linear accelerator. The rounded leaf end MLC design results in an offset between the radiation field edge (the physical leaf position) and the light field (the geometric leaf position). We call this the radiation field offset (RFO). The leaf position is calibrated to the leaf tip at the mid-leaf plane. There is an additional offset between the geometric leaf position and the projected leaf tip position that varies as a function of distance from the collimator central axis due to the MLC geometry. We call this the leaf position offset (LPO). There is a lack of consistency in the interpretation and implementation of the RFO and the LPO in the literature. We investigated the RFO and the LPO on Varian's 600 C/D and 21 EX linear accelerators. We used a combination of film and ion chamber measurements of static, segmental MLC (SMLC) and dynamic MLC (DMLC) fields to quantify the leaf offsets across the range of leaf positions. We were able to improve the dosimetry at large off-axis positions with minor adjustments to the vendor's LPO file. The RFO was determined to within 0.1 mm accuracy at the collimator central axis. The measured RFO value depends on whether the method is based on the radiation field edge position or on an integral dose measurement. The integral dose method results in an RFO that is approximately 0.2 mm greater than the radiation field edge method. The difference is due to the MLC penumbra shape. We propose a methodology for measuring and implementing MLC leaf offsets that is suitable for both SMLC and DMLC IMRT. In addition, we propose some definitions that more clearly describe the MLC leaf position for accurate IMRT dosimetry.