Abuhaimed Abdullah, Martin Colin J, Sankaralingam Marimuthu, Gentle David J
Radiotherapy Physics, Department of Clinical Physics and Bioengineering, Beatson West of Scotland Cancer Centre, Glasgow, UK. Department of Clinical Physics, University of Glasgow, Glasgow, UK. Department of Applied Physics, King Abdulaziz City for Science and Technology, Riyadh, Saudi Arabia.
Phys Med Biol. 2015 Feb 21;60(4):1519-42. doi: 10.1088/0031-9155/60/4/1519. Epub 2015 Jan 23.
Many studies have shown that the computed tomography dose index (CTDI100) which is considered as a main dose descriptor for CT dosimetry fails to provide a realistic reflection of the dose involved in cone beam computed tomography (CBCT) scans. Several practical approaches have been proposed to overcome drawbacks of the CTDI100. One of these is the cumulative dose concept. The purpose of this study was to investigate four different approaches based on the cumulative dose concept: the cumulative dose (1) f(0,150) and (2) f(0,∞) with a small ionization chamber 20 mm long, and the cumulative dose (3) f100(150) and (4) f100(∞) with a standard 100 mm pencil ionization chamber. The study also aimed to investigate the influence of using the 20 and 100 mm chambers and the standard and the infinitely long phantoms on cumulative dose measurements. Monte Carlo EGSnrc/BEAMnrc and EGSnrc/DOSXYZnrc codes were used to simulate a kV imaging system integrated with a TrueBeam linear accelerator and to calculate doses within cylindrical head and body PMMA phantoms with diameters of 16 cm and 32 cm, respectively, and lengths of 150, 600, 900 mm. f(0,150) and f100(150) approaches were studied within the standard PMMA phantoms (150 mm), while the other approaches f(0,∞) and f100(∞) were within infinitely long head (600 mm) and body (900 mm) phantoms. CTDI∞ values were used as a standard to compare the dose values for the approaches studied at the centre and periphery of the phantoms and for the weighted values. Four scanning protocols and beams of width 20-300 mm were used. It has been shown that the f(0,∞) approach gave the highest dose values which were comparable to CTDI∞ values for wide beams. The differences between the weighted dose values obtained with the 20 and 100 mm chambers were significant for the beam widths <120 mm, but these differences declined with increasing beam widths to be within 4%. The weighted dose values calculated within the infinitely long phantoms with both the chambers for the beam widths ≤140 were within 3% of those within the standard phantoms, but the differences rose to be within 15% at wider beams. By comparing the approaches studied in this investigation with other methodologies taking into account the efficiency of the approach as a dose descriptor and the simplicity of the implementation in the clinical environment, the f(0,150) method may be the best for CBCT dosimetry combined with the use of correction factors.
许多研究表明，被视为CT剂量测定主要剂量描述符的计算机断层扫描剂量指数（CTDI100）无法真实反映锥束计算机断层扫描（CBCT）扫描所涉及的剂量。已经提出了几种实用方法来克服CTDI100的缺点。其中之一是累积剂量概念。本研究的目的是研究基于累积剂量概念的四种不同方法：使用20毫米长的小型电离室的累积剂量（1）f(0,150)和（2）f(0,∞)，以及使用标准的100毫米铅笔电离室的累积剂量（3）f100(150)和（4）f100(∞)。该研究还旨在研究使用20毫米和100毫米电离室以及标准和无限长体模对累积剂量测量的影响。使用蒙特卡罗EGSnrc/BEAMnrc和EGSnrc/DOSXYZnrc代码来模拟与TrueBeam直线加速器集成的kV成像系统，并计算分别具有16厘米和32厘米直径以及150、600、900毫米长度的圆柱形头部和身体PMMA体模内的剂量。在标准PMMA体模（150毫米）内研究f(0,150)和f100(150)方法，而其他方法f(0,∞)和f100(∞)在无限长的头部（600毫米）和身体（900毫米）体模内研究。CTDI∞值用作标准，以比较在所研究方法在体模中心和周边以及加权值处的剂量值。使用了四种扫描协议和宽度为20 - 300毫米的射束。结果表明，f(0,∞)方法给出的剂量值最高，对于宽射束，该值与CTDI∞值相当。对于宽度<120毫米的射束，使用20毫米和100毫米电离室获得的加权剂量值之间的差异显著，但随着射束宽度增加，这些差异下降至4%以内。对于宽度≤140的射束，使用两种电离室在无限长体模内计算的加权剂量值与标准体模内的加权剂量值相差在3%以内，但在更宽射束时，差异上升至15%以内。通过将本研究中所研究的方法与其他方法进行比较，同时考虑到该方法作为剂量描述符的效率以及在临床环境中实施的简便性，f(0,150)方法结合使用校正因子可能是CBCT剂量测定的最佳方法。
