Department of Medical Physics, Odette Cancer Centre, Toronto, Ontario, Canada.
J Appl Clin Med Phys. 2010 Jun 15;11(3):3105. doi: 10.1120/jacmp.v11i3.3105.
In this work, leakage radiation from EA200 series electron applicators on Siemens Primus accelerators is quantified, and its penetration ability in water and/or the shielding material Xenolite-NL established. Initially, measurement of leakage from 10 x 10 - 25 x 25 cm2 applicators was performed as a function of height along applicator and of lateral distance from applicator body. Relative to central-axis ionization maximum in solid water, the maximum leakage in air observed with a cylindrical ion chamber with 1 cm solid water buildup cap at a lateral distance of 2 cm from the front and right sidewalls of applicators were 17% and 14%, respectively; these maxima were recorded for 18 MeV electron beams and applicator sizes of >or=20 x 20 cm2. In the patient plane, the applicator leakage gave rise to a broad peripheral dose off-axis distance peak that shifted closer to the field edge as the electron energy increases. The maximum peripheral dose from normally incident primary electron beams at a depth of 1 cm in a water phantom was observed to be equal to 5% of the central-axis dose maximum and as high as 9% for obliquely incident beams with angles of obliquity <or=40 degrees . Measured depth-peripheral dose curves showed that the "practical range" of the leakage electrons in water varies from approximately 1.4 to 5.7 cm as the primary electron beam energy is raised from 6 to 18 MeV. Next, transmission measurements of leakage radiation through the shielding material Xenolite-NL showed a 4 mm thick sheet of this material is required to attenuate the leakage from 9 MeV beams by two-thirds, and that for every additional 3 MeV increase in the primary electron beam energy, an additional Xenolite-NL thickness of roughly 2 mm is needed to achieve the aforementioned attenuation level. Finally, attachment of a 1 mm thick sheet of lead to the outer surface of applicator sidewalls resulted in a reduction of the peripheral dose by up to 80% and 74% for 9 and 18MeV beams, respectively. This sidewall modification had an insignificant effect on the clinical depth dose, cross-axis beam profiles, and output factors.
在这项工作中，定量评估了西门子 Primus 加速器上的 EA200 系列电子施源器的漏射线，并确定了其在水中和/或屏蔽材料 Xenolite-NL 中的穿透能力。首先，测量了 10×10-25×25cm2 施源器沿施源器高度和离施源器体横向距离的漏射线。在离前侧壁和右侧壁 2cm 处，用一个带有 1cm 固体水帽的圆柱形离子室测量，与固体水中心轴离子最大相比，在 18MeV 电子束和施源器尺寸大于等于 20×20cm2 时，观察到的最大空气漏射线分别为 17%和 14%；这些最大值是在 18MeV 电子束下记录的。在患者平面，施源器漏射线引起离轴距离宽的周围剂量峰值，随着电子能量增加，该峰值向射野边缘移动。在水模体中深度为 1cm 处，垂直入射初级电子束的最大周围剂量与中心轴剂量最大值的 5%相等，而斜入射角度小于等于 40 度的束最大周围剂量高达 9%。测量的深度-周围剂量曲线表明，随着电子束能量从 6MeV 提高到 18MeV，漏射线电子在水中的“实际射程”从大约 1.4cm 变化到 5.7cm。接下来，通过 Xenolite-NL 屏蔽材料的透射测量表明，需要 4mm 厚的这种材料才能将 9MeV 束的漏射线衰减三分之二，并且对于初级电子束能量每增加 3MeV，就需要大约 2mm 厚的 Xenolite-NL 来达到上述衰减水平。最后，在施源器侧壁外表面附加 1mm 厚的铅片，可使 9MeV 和 18MeV 束的周围剂量分别降低 80%和 74%。这种侧壁修改对临床深度剂量、交叉轴束轮廓和输出因子没有显著影响。
