Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD, 21201, USA.
Med Phys. 2018 Jul;45(7):3246-3257. doi: 10.1002/mp.12939. Epub 2018 May 17.
Although small animal image-guided radiotherapy (SA-IGRT) systems are used increasingly in preclinical research, tools for performing routine quality assurance (QA) have not been optimized and are not readily available. Robust, efficient, and reliable QA tools are needed to ensure the accuracy and reproducibility of SA-IGRT systems. Several investigators have reported custom-made phantoms and protocols for SA-IGRT systems QA. These are typically time and resource intensive and are therefore not well suited to the preclinical radiotherapy environment, in which physics support is limited and routine QA is performed by technical staff. We investigated the use of the inbuilt electronic portal imaging device (EPID) to develop and validate routine QA tests and procedures. In this work, we focus on the Xstrahl Small Animal Radiation Research Platform (SARRP) EPID. However, the methodology and tests developed here are applicable to any SA-IGRT system that incorporates an EPID.
We performed a comprehensive characterization of the dosimetric properties of the camera-based EPID at kilovoltage energies over a 11-month period, including detector warm-up time, radiation dose history effect, stability and short- and long-term reproducibility, gantry angle dependency, output factor, and linearity of the EPID response. We developed a test to measure the constancy of beam quality in terms of half-value layer and tube peak potential using the EPID. We verified the SARRP daily output and beam profile constancy using the imager. We investigated the use of the imager to monitor beam-targeting accuracy at various gantry and couch angles.
The EPID response was stable and reproducible, exhibiting maximum variations of ≤0.3% and ≤1.9% for short and long terms, respectively. The detector showed no dependence on response at different gantry angles, with a maximum variation ≤0.5%. We found close agreement in output factor measurement between the portal imager and reference dosimeters, with maximum differences ≤3% for ionization chamber and ≤1.7% for Gafchromic EBT3 dosimetry film, respectively. We have shown that the EPID response is linear with tube current (mA) for the entire range of tube kilovoltage peak. Notably, a close relationship was seen between the detector response vs mA slope, and the kilovoltage peak, allowing an independent verification of kilovoltage peak stability based solely on EPID response. In addition to dosimetry tests, according to the beam-targeting measurement using portal images, maximum displacement of the central axis of the x-ray beam (due to sag) was 0.76 ± 0.09 mm at gantry 135°/couch 0° and 0.89 ± 0.06 mm at gantry 0°/couch -135°.
We performed the first comprehensive analysis on the dosimetric properties of an EPID operating at kilovoltage x-ray energies. We characterized the detector performance over a 11-month period. Our results indicate that the imager is a stable and convenient tool for SARRP routine QA tests. We then developed EPID-based tests to perform routine SA-IGRT systems QA tasks, such as verifying constancy of beam quality, energy, output, and profile measurements, relative output factors, and beam targeting.
尽管小动物图像引导放疗（SA-IGRT）系统在临床前研究中越来越多地被使用，但用于执行常规质量保证（QA）的工具尚未得到优化，也不容易获得。需要稳健、高效和可靠的 QA 工具来确保 SA-IGRT 系统的准确性和可重复性。一些研究人员已经报告了用于 SA-IGRT 系统 QA 的定制化体模和方案。这些通常既耗时又耗资源，因此不太适合临床前放疗环境，在该环境中，物理支持有限，并且由技术人员执行常规 QA。我们研究了使用内置电子门户成像设备（EPID）来开发和验证常规 QA 测试和程序。在这项工作中，我们专注于 Xstrahl 小动物辐射研究平台（SARRP）的 EPID。然而，开发的方法和测试适用于任何包含 EPID 的 SA-IGRT 系统。
我们在 11 个月的时间内对基于相机的 EPID 在千伏能量下的剂量学特性进行了全面表征，包括探测器预热时间、辐射剂量历史效应、稳定性和短期及长期重复性、机架角度依赖性、输出因子以及 EPID 响应的线性度。我们开发了一种使用 EPID 测量半值层和管峰值电位的光束质量恒定性的测试。我们使用成像仪验证了 SARRP 的日常输出和射束轮廓恒定性。我们研究了在不同机架和治疗床角度下使用成像仪监测射束靶向准确性的用途。
EPID 响应稳定且可重复，在短期和长期分别表现出最大变化≤0.3%和≤1.9%。探测器在不同机架角度下的响应没有依赖性，最大变化≤0.5%。我们发现，通过门户成像仪和参考剂量计测量输出因子之间存在很好的一致性，电离室最大差异≤3%，Gafchromic EBT3 剂量测定胶片最大差异≤1.7%。我们已经表明，EPID 响应与管电流（mA）在管千伏峰值的整个范围内呈线性关系。值得注意的是，在探测器响应与 mA 斜率之间存在密切关系，以及千伏峰值，这允许仅基于 EPID 响应独立验证千伏峰值稳定性。除了剂量学测试外，根据使用门户图像的射束靶向测量，在机架 135°/治疗床 0°和机架 0°/治疗床-135°时，X 射线束的中心轴（由于下垂）的最大位移为 0.76±0.09mm 和 0.89±0.06mm。
我们对在千伏 X 射线能量下运行的 EPID 的剂量学特性进行了首次全面分析。我们在 11 个月的时间内对探测器性能进行了表征。我们的结果表明，成像仪是 SARRP 常规 QA 测试的稳定、便捷工具。然后，我们开发了基于 EPID 的测试，以执行常规的 SA-IGRT 系统 QA 任务，例如验证光束质量、能量、输出和射束轮廓测量、相对输出因子以及射束靶向的恒定性。
