Hoffman David, Chung Eunah, Hess Clayton, Stern Robin, Benedict Stanley
Department of Radiation Medicine and Applied Sciences, University of California, San Diego, CA, USA.
Department of Radiation Oncology, Samsung Medical Center, Seoul, South Korea.
J Appl Clin Med Phys. 2017 Jan;18(1):40-48. doi: 10.1002/acm2.12014. Epub 2016 Nov 29.
The aim of this work was to comprehensively evaluate a new large field ion chamber transmission detector, Integral Quality Monitor (IQM), for online external photon beam verification and quality assurance. The device is designed to be mounted on the linac accessory tray to measure and verify photon energy, field shape, gantry position, and fluence before and during patient treatment.
Our institution evaluated the newly developed ion chamber's effect on photon beam fluence, response to dose, detection of photon fluence modification, and the accuracy of the integrated barometer, thermometer, and inclinometer. The detection of photon fluence modifications was performed by measuring 6 MV with fields of 10 cm × 10 cm and 1 cm × 1 cm "correct" beam, and then altering the beam modifiers to simulate minor and major delivery deviations. The type and magnitude of the deviations selected for evaluation were based on the specifications for photon output and MLC position reported in AAPM Task Group Report 142. Additionally, the change in ion chamber signal caused by a simulated IMRT delivery error is evaluated.
The device attenuated 6 MV, 10 MV, and 15 MV photon beams by 5.43 ± 0.02%, 4.60 ± 0.02%, and 4.21 ± 0.03%, respectively. Photon beam profiles were altered with the IQM by < 1.5% in the nonpenumbra regions of the beams. The photon beam profile for a 1 cm × 1 cm fields were unchanged by the presence of the device. The large area ion chamber measurements were reproducible on the same day with a 0.14% standard deviation and stable over 4 weeks with a 0.47% SD. The ion chamber's dose-response was linear (R = 0.99999). The integrated thermometer agreed to a calibrated thermometer to within 1.0 ± 0.7°C. The integrated barometer agreed to a mercury barometer to within 2.3 ± 0.4 mmHg. The integrated inclinometer gantry angle measurement agreed with the spirit level at 0 and 180 degrees within 0.03 ± 0.01 degrees and 0.27 ± 0.03 at 90 and 270 degrees. For the collimator angle measurement, the IQM inclinometer agreed with a plum-bob within 0.3 ± 0.2 degrees. The simulated IMRT error increased the ion chamber signal by a factor of 11-238 times the baseline measurement for each segment.
The device signal was dependent on variations in MU delivered, field position, single MLC leaf position, and nominal photon energy for both the 1 cm × 1 cm and 10 cm × 10 cm fields. This detector has demonstrated utility repeated photon beam measurement, including in IMRT and small field applications.
本研究旨在全面评估一种新型大视野电离室透射探测器——积分质量监测仪（IQM），用于在线外照射光子束验证和质量保证。该设备设计安装在直线加速器附件托盘上，以在患者治疗前和治疗期间测量和验证光子能量、射野形状、机架位置和注量。
我们机构评估了新开发的电离室对光子束注量的影响、对剂量的响应、光子注量修正的检测以及集成气压计、温度计和倾角仪的准确性。通过测量10 cm×10 cm射野和1 cm×1 cm“正确”射野的6 MV光子束，然后改变射束修正器以模拟轻微和严重的射束输出偏差，来进行光子注量修正的检测。选择用于评估的偏差类型和幅度基于AAPM任务组报告142中报告的光子输出和多叶准直器（MLC）位置的规范。此外，还评估了由模拟调强放射治疗（IMRT）输出误差引起的电离室信号变化。
该设备对6 MV、10 MV和15 MV光子束的衰减分别为5.43±0.02%、4.60±0.02%和4.21±0.03%。在射束的非半影区域，IQM对光子束轮廓的改变小于1.5%。对于1 cm×1 cm的射野，该设备的存在未改变光子束轮廓。大面积电离室测量在同一天的重复性标准差为0.14%，在4周内稳定，标准差为0.47%。电离室的剂量响应呈线性（R = 0.99999）。集成温度计与校准温度计的一致性在1.0±0.7°C以内。集成气压计与水银气压计的一致性在2.3±0.4 mmHg以内。集成倾角仪对机架角度的测量在0°和180°时与水平仪的一致性在0.03±0.01°以内，在90°和270°时在0.27±0.03°以内。对于准直器角度测量，IQM倾角仪与铅垂线的一致性在0.3±0.2°以内。模拟的IMRT误差使电离室信号在每个子野的基线测量值基础上增加了11 - 238倍。
对于1 cm×1 cm和10 cm×10 cm的射野，该设备的信号取决于所输送的监测单位（MU）、射野位置、单个MLC叶片位置和标称光子能量的变化。该探测器已证明可用于重复的光子束测量，包括在IMRT和小射野应用中。
