Partridge M, Evans P M, Mosleh-Shirazi M A
Joint Department of Physics, Institute of Cancer Research, Royal Marsden NHS Trust, Sutton, United Kingdom.
Med Phys. 1998 Aug;25(8):1443-52. doi: 10.1118/1.598318.
An experimental study of radiation output intensity fluctuations of a Philips SL25 linear accelerator is presented. Measurements are obtained using an electronic portal imaging device, and the consequences of the measured fluctuations for various different applications of megavoltage imaging including portal imaging, transit dosimetry and megavoltage computed tomography (MVCT) are discussed with examples. Fluctuations in output of +/- 0.7% (1 SD) are seen on every radiation pulse after photon noise and uncertainties caused by the detection system have been accounted for. Large fluctuations are also seen during the initial beam stabilization period (15%), during normal accelerator operation after the beam has been on for more than 1 min (4.5%) and during are therapy as a repeatable function of gantry angle (9%). Such output intensity fluctuations are shown to produce image artifacts in portal imaging devices with scanned detector readout and can also produce systematic errors in detector calibration that would lead to uncertainty in transit dose calculations. The propagation of these intensity fluctuations through MVCT image reconstruction is shown to produce ring artifacts in the reconstructed image. Sample portal and MVCT images are presented. All observed fluctuations in accelerator output are well within the manufacturer's specifications and do not affect the total dose delivered during normal treatment. Finally, megavoltage imaging is shown to be a powerful tool for accelerator quality assurance and treatment verification.
本文介绍了一项关于飞利浦SL25直线加速器辐射输出强度波动的实验研究。使用电子射野成像设备进行测量，并通过实例讨论了测量波动对兆伏级成像的各种不同应用（包括射野成像、传输剂量测定和兆伏级计算机断层扫描（MVCT））的影响。在考虑了光子噪声和检测系统引起的不确定性之后，在每个辐射脉冲上都观察到了±0.7%（1标准差）的输出波动。在初始束流稳定期（15%）、束流开启超过1分钟后的正常加速器运行期间（4.5%）以及放射治疗期间作为机架角度的可重复函数（9%）也观察到了较大的波动。这种输出强度波动在具有扫描探测器读出的射野成像设备中会产生图像伪影，并且在探测器校准中也会产生系统误差，这将导致传输剂量计算的不确定性。这些强度波动通过MVCT图像重建的传播在重建图像中会产生环形伪影。给出了示例射野图像和MVCT图像。加速器输出中观察到的所有波动均完全在制造商的规格范围内，并且不影响正常治疗期间输送的总剂量。最后，兆伏级成像被证明是加速器质量保证和治疗验证的有力工具。
