The National Centre for Applied Physics, King Abdulaziz City for Science and Technology (KACST), Riyadh, Saudi Arabia. Department of Biomedical Physics, King Faisal Specialist Hospital and Research Centre, Riyadh, Saudi Arabia. Author to whom any correspondence should be addressed.
Phys Med Biol. 2019 May 23;64(11):115002. doi: 10.1088/1361-6560/ab0bc8.
The CT dose index (CTDI) is the dosimetric quantity used for multi-slice CT (MSCT) with beams ⩽4 cm. Conversion factors (f ) based on patient size are applied to CTDI to adjust for differences in patient size and derive size-specific dose estimates (SSDE) relating to patient dose. The aim of this study is to: (1) investigate use of a similar technique to provide SSDE values for cone beam CT (CBCT) scans, (2) determine whether factors derived for narrow beam MSCT are suitable for CBCT, and (3) investigate the influence of CBCT parameters on f values. Monte Carlo simulations were used to model an on-board imager system integrated into a Varian Truebeam linear accelerator, and to assess doses for imaging. The CTDI is unsuitable for CBCT dosimetry, thus the assessments were applied to the modification CTDI, recommended by the international electrotechnical commission (IEC). Conversion factors (f ) were derived for CBCT to allow adjustment of values for the absorbed dose in water (AD) averaged over the beam width in the middle of the phantoms. Values of AD were evaluated as for CTDI measured with a 30 cm long chamber in centres and peripheries of long water phantoms with diameters of 10-40 cm for head and body protocols. Three beam widths 8, 16, and 24 cm were used with tube potentials ranging from 80-140 kV, for full and partial rotation modes. In order to derive f values, calculated values for AD were normalized with respect to the weighted CTDI assessed in standard CTDI phantoms and free-in-air. Variations in f with beam width were minimal, 1%-5%, but those with tube potential were greater for 80 kV at small diameters reaching 11%. Acquisition mode affected f values by up to 7%. Best-fit curves were derived from the f values and compared to those reported by AAPM TG-204 for MSCT. Conversion factors estimated from these curves for 120 kV were within ±8% and ±13% of the MSCT values over head and body diameters, respectively, representing the majority of the adult population. Therefore, the use of MSCT factors to convert CTDI for CBCT scans may lead to under/overestimation of doses to patients by 5% and 6%, on average, for the head and body protocols, respectively. Best-fit curves of the results from this study provide values that could be used to convert CTDI for specific water-equivalent diameter (D ) to a SSDE.
CT 剂量指数（CTDI）是用于束宽≤4cm 的多层 CT（MSCT）的剂量学量。基于患者体型的转换因子（f）被应用于 CTDI，以调整患者体型的差异，并得出与患者剂量相关的体型特异性剂量估计值（SSDE）。本研究的目的是：（1）研究使用类似技术为锥形束 CT（CBCT）扫描提供 SSDE 值，（2）确定为窄束 MSCT 推导的因子是否适用于 CBCT，以及（3）研究 CBCT 参数对 f 值的影响。使用蒙特卡罗模拟来模拟集成到瓦里安 Truebeam 直线加速器中的机载成像系统，并评估成像剂量。CTDI 不适合 CBCT 剂量测量，因此评估应用于国际电工委员会（IEC）推荐的修改 CTDI。为了调整光束宽度中间的水吸收剂量（AD）平均值的 CBCT 的值，推导了转换因子（f）。AD 值的评估与在长水模体的中心和外围用 30cm 长的腔室测量的 CTDI 相同，长水模体的直径为 10-40cm，用于头部和身体协议。使用 8、16 和 24cm 三种光束宽度，并在 80-140kV 的管电压下，在全旋转和部分旋转模式下进行测量。为了推导 f 值，用标准 CTDI 模体和自由空气中评估的加权 CTDI 对计算的 AD 值进行归一化。f 值随光束宽度的变化很小，为 1%-5%，但随管电压的变化较大，对于小直径的 80kV 可达 11%。采集模式对 f 值的影响最大可达 7%。从 f 值中推导出最佳拟合曲线，并与 AAPM TG-204 为 MSCT 报告的曲线进行比较。对于 120kV，从这些曲线估计的转换因子在头部和身体直径处分别为 MSCT 值的±8%和±13%以内，代表了大多数成年人群。因此，在为 CBCT 扫描转换 CTDI 时，使用 MSCT 因子可能会导致头部和身体协议的患者剂量低估/高估平均 5%和 6%。本研究结果的最佳拟合曲线提供了可以用于将特定水等效直径（D）的 CTDI 转换为 SSDE 的值。
