Medical Physics Unit, McGill University, Montréal, Québec H3G 1A4, Canada.
Med Phys. 2010 Mar;37(3):1083-92. doi: 10.1118/1.3302140.
A common approach for dose assessment during cone beam computed tomography (CBCT) acquisition is to use thermoluminescent detectors for skin dose measurements (on patients or phantoms) or ionization chamber (in phantoms) for body dose measurements. However, the benefits of a daily CBCT image acquisition such as margin reduction in planning target volume and the image quality must be weighted against the extra dose received during CBCT acquisitions.
The authors describe a two-dimensional reference dosimetry technique for measuring dose from CBCT scans using the on-board imaging system on a Varian Clinac-iX linear accelerator that employs the XR-QA radiochromic film model, specifically designed for dose measurements at low energy photons. The CBCT dose measurements were performed for three different body regions (head and neck, pelvis, and thorax) using humanoid Rando phantom.
The authors report on both surface dose and dose profiles measurements during clinical CBCT procedures carried out on a humanoid Rando phantom. Our measurements show that the surface doses per CBCT scan can range anywhere between 0.1 and 4.7 cGy, with the lowest surface dose observed in the head and neck region, while the highest surface dose was observed for the Pelvis spot light CBCT protocol in the pelvic region, on the posterior side of the Rando phantom. The authors also present results of the uncertainty analysis of our XR-QA radiochromic film dosimetry system.
Radiochromic film dosimetry protocol described in this work was used to perform dose measurements during CBCT acquisitions with the one-sigma dose measurement uncertainty of up to 3% for doses above 1 cGy. Our protocol is based on film exposure calibration in terms of "air kerma in air," which simplifies both the calibration procedure and reference dosimetry measurements. The results from a full Monte Carlo investigation of the dose conversion of measured XR-QA film dose at the surface into dose to water (or water kerma) at the surface of the phantom indicate that, for typical beam qualities used in CBCT, this conversion can be approximated by simple mass-energy absorption coefficient ratios water-to-air.
在锥形束计算机断层扫描（CBCT）采集期间进行剂量评估的一种常见方法是使用热释光探测器进行皮肤剂量测量（在患者或体模上）或电离室（在体模中）进行体部剂量测量。然而，必须权衡每日 CBCT 图像采集的好处，例如计划靶区的边缘减少和图像质量，与 CBCT 采集期间额外接受的剂量。
作者描述了一种二维参考剂量测量技术，用于使用瓦里安 Clinac-iX 直线加速器上的机载成像系统测量 CBCT 扫描的剂量，该系统采用 XR-QA 光致变色胶片模型，专门设计用于低能光子的剂量测量。使用仿人 Rando 体模对三个不同的身体区域（头颈部、骨盆和胸部）进行了 CBCT 剂量测量。
作者报告了在仿人 Rando 体模上进行的临床 CBCT 程序中进行的表面剂量和剂量分布测量。我们的测量结果表明，每次 CBCT 扫描的表面剂量范围在 0.1 至 4.7 cGy 之间，头颈部的表面剂量最低，而骨盆区域的骨盆点光 CBCT 协议的表面剂量最高，位于 Rando 体模的后侧。作者还介绍了我们的 XR-QA 光致变色胶片剂量测量系统的不确定度分析结果。
本工作中描述的光致变色胶片剂量测量方案用于在 CBCT 采集期间进行剂量测量，对于超过 1 cGy 的剂量，一sigma 剂量测量不确定度高达 3%。我们的方案基于“空气中的空气比释动能”的胶片曝光校准，简化了校准程序和参考剂量测量。通过对测量的 XR-QA 胶片剂量在表面处转换为体模表面处的水剂量（或水比释动能）的全面蒙特卡罗研究的结果表明，对于 CBCT 中使用的典型束质，这种转换可以通过简单的质量-能量吸收系数比水-空气来近似。
