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首页> 外文期刊>British Journal of Radiology >Simple methods to reduce patient dose in a Varian cone beam CT system for delivery verification in pelvic radiotherapy.
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Simple methods to reduce patient dose in a Varian cone beam CT system for delivery verification in pelvic radiotherapy.

机译：在Varian锥形束CT系统中减少患者剂量的简单方法，用于盆腔放疗中的传递验证。

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摘要

Cone-beam computed tomography (CBCT) is a three-dimensional imaging modality that has recently become available on linear accelerators for radiotherapy patient position verification. It was the aim of the present study to implement simple strategies for reduction of the dose delivered in a commercial CBCT system. The dose delivered in a CBCT procedure (Varian, half-fan acquisition, 650 projections, 125 kVp) was assessed using a cylindrical Perspex phantom (diameter, 32 cm) with a calibrated Farmer type ionisation chamber. A copper filter (thickness, 0.15 mm) was introduced increasing the half value layer of the beam from 5.5 mm Al to 8 mm Al. Image quality and noise were assessed using an image quality phantom (CatPhan) while the exposure settings per projection were varied from 25 ms/80 mA to 2 ms/2 mA per projection. Using the copper filter reduced the dose to the phantom from approximately 45 mGy to 30 mGy at standard settings (centre/periphery weighting 1/3 to 2/3). Multiple CBCT images were acquired for six patients with pelvic malignancies to compare CBCTs with and without a copper filter. Although the reconstructed image is somewhat noisier with the filter, it features similar contrast in the centre of the patient and was often preferred by the radiation oncologist because of greater image uniformity. The X-ray shutters were adjusted to the minimum size required to obtain the desired image volume for a given patient diameter. The simple methods described here reduce the effective dose to patients undergoing daily CBCT and are easy to implement, and initial evidence suggests that they do not affect the ability to identify soft tissue for the purpose of treatment verification.
机译：锥形束计算机断层扫描（CBCT）是一种三维成像模式，最近已在线性加速器上用于放射治疗患者位置验证。本研究的目的是实施减少商业CBCT系统中递送剂量的简单策略。使用带有校准的Farmer型电离室的圆柱形有机玻璃体模（直径32 cm）评估CBCT程序中的剂量（瓦里安，半风扇采集，650个投影，125 kVp）。引入了铜滤光片（厚度为0.15 mm），使光束的半值层从5.5 mm Al增加到8 mm Al。使用图像质量体模（CatPhan）评估图像质量和噪声，同时每个投影的曝光设置在25 ms / 80 mA到2 ms / 2 mA的范围内变化。在标准设置下（中心/外围权重为1/3到2/3），使用铜滤波器可以将幻影的剂量从大约45 mGy减少到30 mGy。为六名骨盆恶性肿瘤患者获取了多个CBCT图像，以比较使用铜过滤器和不使用铜过滤器的CBCT。尽管重建的图像在使用滤镜时会有些吵杂，但它在患者中心具有相似的对比度，并且由于图像的均匀性更高，通常被放射线肿瘤学家所偏爱。将X射线快门调整到为给定患者直径获得所需图像量所需的最小尺寸。这里描述的简单方法可以降低每天进行CBCT的患者的有效剂量，并且易于实施，并且初步证据表明它们不影响以治疗验证为目的识别软组织的能力。

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