Abuhaimed Abdullah, Martin Colin J
The National Centre for Applied Physics, King Abdulaziz City for Science and Technology, Riyadh, Saudi Arabia.
J Radiol Prot. 2018 Mar;38(1):189-206. doi: 10.1088/1361-6498/aa9b9f. Epub 2017 Nov 20.
Regular imaging is used throughout image guided radiation therapy to improve treatment delivery. In order for treatment procedures to be optimized, the doses delivered by imaging exposures should be taken into account. CT dosimetry methods based on the CT dose index (CTDI), measured with a 100 mm long pencil ionization chamber (CTDI) in standard phantoms, are not designed for cone-beam CT (CBCT) imaging systems used in radiotherapy, therefore a modified version has been proposed for CBCT by the International Electrotechnical Commission (CTDI). Monte Carlo simulations based on a Varian On-Board Imaging system were used to derive conversion coefficients that enable organ doses for ICRP reference phantoms to be determined from the CTDI for different scan protocols and different beam widths (80-320) mm. A dose-width product calculated by multiplying the CTDI by the width of the CBCT beam is proposed as a quantity that can be used for estimating effective dose. The variation in coefficients with CBCT beam width was studied. Coefficients to allow estimation of effective doses were derived, namely 0.0034 mSv (mGy cm) for the head, 0.0252 mSv (mGy cm) for the thorax, 0.0216 mSv (mGy cm) for the abdomen and 0.0150 mSv (mGy cm) for the pelvis, and these may be applicable more generally to other CBCT systems in radiotherapy. If data on effective doses are available, these can be used in making judgements on the contributions to patient dose from imaging, and thereby assist in optimization of the treatment regimes. The coefficients can also be employed in converting dosimetry data recorded in patient records into quantities relating directly to patient doses.
在图像引导放射治疗中，常规成像用于改善治疗实施。为了优化治疗程序，应考虑成像曝光所输送的剂量。基于CT剂量指数（CTDI）的CT剂量测定方法，是用100毫米长的铅笔电离室在标准体模中测量的，不适用于放射治疗中使用的锥形束CT（CBCT）成像系统，因此国际电工委员会（IEC）针对CBCT提出了一个改进版本（CTDIvol）。基于Varian机载成像系统的蒙特卡罗模拟被用于推导转换系数，这些系数可根据不同扫描协议和不同束宽（80 - 320）毫米下的CTDIvol确定ICRP参考体模的器官剂量。通过将CTDIvol乘以CBCT束宽计算得到的剂量 - 宽度乘积被提议作为一个可用于估计有效剂量的量。研究了系数随CBCT束宽的变化。推导了用于估计有效剂量的系数，即头部为0.0034 mSv/(mGy·cm)，胸部为0.0252 mSv/(mGy·cm)，腹部为0.0216 mSv/(mGy·cm)，骨盆为0.0150 mSv/(mGy·cm)，这些系数可能更普遍地适用于放射治疗中的其他CBCT系统。如果有有效剂量的数据，这些数据可用于判断成像对患者剂量的贡献，从而有助于优化治疗方案。这些系数还可用于将患者记录中记录的剂量测定数据转换为与患者剂量直接相关的量。
