Parsons David, Stevens M Tynan R, Robar James L
Department of Physics and Atmospheric Science, Dalhousie University, 5820 University Avenue, Halifax, Nova Scotia, B3H 1V7, Canada.
Department of Radiation Oncology and Department of Physics and Atmospheric Science, Dalhousie University, 5820 University Avenue, Halifax, Nova Scotia, B3H 1V7, Canada.
Med Phys. 2017 Apr;44(4):1479-1493. doi: 10.1002/mp.12136. Epub 2017 Mar 22.
The focus of this work was to improve the available kV image quality for continuous intrafraction monitoring of the prostate during volumetric modulated arc therapy. This is investigated using a novel blade collimation system enabling tube current modulated (TCM) volume-of-interest (VOI) imaging of prostate fiducial markers during radiotherapy, and Monte Carlo simulation of MV scatter.
A four-blade dynamic kV collimator was used to track a VOI containing gold fiducial markers embedded in a dynamic pelvis phantom during gantry rotation. For each fiducial, a VOI margin around each marker was set to be 2σ of the population covariance matrix characterizing prostate motion. This was used to conform to a single or several fiducials and compared to a static field. DRRs were used to calculate the kV attenuation for each VOI as a function of angle and used to optimize x-ray tube current during acquisition. Image quality was assessed with regard to contrast-to-noise ratio (CNR), fiducial detectability, and imaging dose. Monte Carlo simulations in EGSnrc were used to calculate the imaging dose to the phantom and MV scatter fluence to the imaging panel.
Fiducials can be accurately located using a VOI containing a single or several fiducials using a relatively high constant kV output. However, when using a 6 × 6 cm field the dose can be upwards of 1.5 Gy in bone for constant kV output and 3.1 Gy when applying TCM at 1 Hz imaging over the course of 40 fractions. This can be mitigated through tailoring the imaging field to a single or several fiducials, in which the integral dose is reduced by a factor of 15.6 and 3.7, respectively. For a constant MV treatment field size, the scattered fluence reaching the kV panel varies by less than a factor of two for a completely rotation of the gantry. However, the MV scatter spectrum overlaps with the detector response for a deleterious effect, with a peak MV scatter energy of approximately 100 keV. TCM can be used to overcome the variability in image quality throughout the rotation and therefore improve fiducial CNR and detectability during periods of high kV attenuation.
The combination of VOI and TCM introduces an advantageous approach in intrafraction monitoring of the prostate during radiotherapy by both reducing and localizing the imaging dose, while improving image quality and fiducial detectability during periods of high kV attenuation. In addition, the influence of MV scatter has been shown to be most important in low attenuation regions, with a variation by a factor of two.
本研究的重点是在容积调强弧形放疗期间改善用于前列腺连续分次内监测的千伏（kV）图像质量。使用一种新型叶片准直系统对此进行研究，该系统能够在放射治疗期间对前列腺基准标记物进行管电流调制（TCM）的感兴趣区（VOI）成像，并对兆伏（MV）散射进行蒙特卡罗模拟。
使用四叶片动态kV准直器在机架旋转期间跟踪包含嵌入动态骨盆体模中的金基准标记物的VOI。对于每个基准标记物，围绕每个标记物的VOI边界设置为表征前列腺运动的总体协方差矩阵的2σ。这用于适配单个或多个基准标记物，并与静态野进行比较。数字重建射线影像（DRRs）用于计算每个VOI的kV衰减作为角度的函数，并用于在采集期间优化X射线管电流。从对比度噪声比（CNR）、基准标记物可检测性和成像剂量方面评估图像质量。在EGSnrc中进行的蒙特卡罗模拟用于计算体模的成像剂量和成像面板的MV散射注量。
使用相对较高的恒定kV输出，通过包含单个或多个基准标记物的VOI可以准确地定位基准标记物。然而，当使用6×6 cm野时，对于恒定kV输出，骨内剂量可能超过1.5 Gy，在40次分次过程中以1 Hz成像应用TCM时剂量为3.1 Gy。这可以通过将成像野适配到单个或多个基准标记物来减轻，其中积分剂量分别降低15.6倍和3.7倍。对于恒定的MV治疗野尺寸，在机架完全旋转时，到达kV面板的散射注量变化小于两倍。然而，MV散射谱与探测器响应重叠产生有害影响，MV散射峰值能量约为100 keV。TCM可用于克服整个旋转过程中图像质量的变化，从而在高kV衰减期间提高基准标记物的CNR和可检测性。
VOI和TCM的结合在放射治疗期间对前列腺的分次内监测中引入了一种有利的方法，既减少了成像剂量并使其局部化，同时在高kV衰减期间提高了图像质量和基准标记物的可检测性。此外，已表明MV散射的影响在低衰减区域最为重要，变化幅度为两倍。
