A modification of flattening filter free linac for IMRT
A modification of flattening filter free linac for IMRT
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DOI：
10.1118/1.3571419
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发表时间：
2011-05-01
期刊：
影响因子：
3.8
通讯作者：
Zygmanski, P.
中科院分区：
文献类型：
--
作者：
Tsiamas, P.;Seco, J.;Zygmanski, P.
目的：本研究探讨改良的无平滤(FFF)直线加速器与配备平滤的标准(STD)直线加速器相比的优点。对FFF型直线加速器的电子束能量和角扩散进行了改进。对于大体积或远离中心轴的目标，为了最大化监测单元的效率和最小化头部散射，对FFF波束参数进行了修改。方法：使用EGSnrc程序对FFF和STD直线加速器进行建模，并研究这两种类型的加速器在不同波束配置下的基本波束特性。与STD直线加速器相比，增加FFF直线加速器的能量导致了相似的束流衰减特性和最大剂量率。匹配的束流衰减特性可以更直接地探测FFF直线加速器在调强放射治疗方面的束流平坦度，特别是在远离中心轴的地方，那里的有效剂量率比在中心轴的剂量率小得多。通过增大电子束的角扩散，改善了FFF直线加速器开束剂量分布的平坦度。结果：为了匹配束流穿透特性，小到中型野的能量从6.5 MeV调整到8.0 MeV，大野的能量从6.5 MeV调整到8.5 MeV。FFF与STD直线加速器相比，剂量率分别增加了1.9倍(6.5 MeV)和3.4-4.1倍(8.0-8.5 MeV)。将电子束的平均角扩散从0度调整到5度-10度，导致场大小在10×10厘米(2)到15×15厘米(2)之间的光子束完全平坦，并且场大小从15×15厘米(2)到30×30厘米(2)的部分平坦。角度散布>；=14度的值不被推荐，因为它们超过了主准直器的开口，影响了视野边缘的区域。尺寸小于6×6厘米(2)的FFF场已经是平坦的，并且光束平坦是不必要的。总体而言，5度-10度的角度扩展是足够的，即使对于较大的射野大小，也可以令人满意地平坦开放的束流剂量分布。增大电子束角扩散相当于使FFF型直线加速器的剂量率略有下降。然而，对于角扩散，FFFVS STD的5度-10度剂量率系数仍约为1.6-2.6，这取决于野大小(以及调整后的能量)。同样，在周边剂量的情况下，对于5度-10度的角度扩散和10x10厘米(2)至30x30厘米(2)的野大小，可以观察到适度的剂量增加。最后，未改进的FFF型直线加速器的束流平坦度可以用一个表示STD与FFF剂量之比的解析函数来描述：1+b垂直条r垂直条(N)。结论：改进后的FFF束线具有更大的能量和电子束角扩展，在场内获得了满意的束流平坦度和较高的剂量率。在治疗范围内，对于相同的输送剂量，周围剂量保持在与STD直线加速器相似(或更小)的水平。(C)2011年美国医学物理学家协会。[DOI：10.1118/1.3571419]
Purpose: This study investigates the benefits of a modified flattening filter free (FFF) linac over the standard (STD) linac equipped with the flattening filter. Energy and angular spread of the electron beam of the FFF linac were modified. Modification of FFF beam parameters is explored to maximize the monitor unit efficiency and to minimize the head scatter in IMRT delivery for large target volumes or targets lying away from the central axis.Methods: The EGSnrc code is used to model FFF and STD linacs and study basic beam properties for both linac types in various beam configurations. Increasing energy of FFF linac results in similar beam attenuation properties and maximized dose rate compared to STD linac. Matching beam attenuation properties allows a more direct exploration of beam flatness of FFF linac in regard to IMRT delivery, especially away from the central axis where the effective dose rate is considerably smaller than the one at the central axis. Flatness of open beam dose profile of FFF linac is improved by increasing the angular spread of the electron beam. The resulting dose rate within the treatment field and outside of the field (peripheral dose) are characterized and compared to the unmodified FFF and STD linacs.Results: In order to match beam penetration properties, the energy of FFF is adjusted from 6.5 to 8.0 MeV for small to medium field sizes and from 6.5 to 8.5 MeV for larger ones. Dose rate of FFF vs STD linac increased by a factor of 1.9 (6.5 MeV) and 3.4-4.1 (8.0-8.5 MeV). Adjusting the mean angular spread of the electron beam from 0 degrees to 5 degrees-10 degrees resulted in complete flattening of photon beam for field sizes between 10 x 10 cm(2) and 15 x 15 cm(2) and partial flattening for field sizes from 15 x 15 cm(2) to 30 x 30 cm(2). Values of angular spread >= 14 degrees are not recommended as they exceed the opening of the primary collimator, affecting the area at the edges of the field. FFF fields of sizes smaller than 6 x 6 cm(2) are already flat and beam flattening is not necessary. Overall, the angular spread of 5 degrees-10 degrees is sufficient and can satisfactorily flatten open beam dose profiles even for larger field sizes. Increasing the electron beam angular spread amounts to a slight decrease of dose rate of FFF linac. However, for angular spread, 5 degrees-10 degrees dose rate factor of FFF vs STD is still about 1.6-2.6, depending on the field size (and the adjusted energy). Similarly, in case of peripheral dose, a moderate increase in dose can be observed for angular spread of 5 degrees-10 degrees and for field sizes 10 x 10 cm(2) to 30 x 30 cm(2). Lastly, beam flatness of not modified FFF linac can be conveniently described by an analytical function representing a ratio of STD vs FFF doses: 1 + b vertical bar r vertical bar(n).Conclusions: A modified FFF beamline with increased energy and electron beam angular spread results in satisfactory flattened beam and high dose rate within the field. Peripheral dose remaining at similar (or smaller) level than that of STD linac for the same delivered dose within the treatment field. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3571419]