Daily output variations of up to ±2% were observed for a protracted time on a Varian TrueBeam® STx; these output variations were hypothesized to be the result of atmospheric communication of the sealed monitor chamber. Daily changes in output relative to baseline, measured with an ionization chamber array (DQA3) and the amorphous silicon flat panel detector (IDU) on the TrueBeam®, were compared with daily temperature‐pressure corrections (P_TP) determined from sensors within the DQA3. Output measurements were performed using a Farmer® ionization chamber over a 5‐hour period, during which there was controlled variation in the monitor chamber temperature. The root mean square difference between percentage output change from baseline measured with the DQA3 and IDU was 0.50% over all measurements. Over a 7‐month retrospective review of daily changes in output and P_TP, weak correlation (R² = 0.30) was observed between output and P_TP for the first 5 months; for the final 2 months, daily output changes were linearly correlated with changes in P_TP, with a slope of 0.84 (R² = 0.89). Ionization measurements corrected for ambient temperature and pressure during controlled heating and cooling of the monitor chamber differed from expected values for a sealed monitor chamber by up to 4.6%, but were consistent with expectation for an air‐communicating monitor chamber within uncertainty (1.3%, k = 2). Following replacement of the depressurized monitor chamber, there has been no correlation between daily percentage change in output and P_TP (R² = 0.09). The utility of control charts is demonstrated for earlier identification of changes in the sensitivity of a sealed monitor chamber.

中文翻译：

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诊断密封的监控室的大气连通性。

在VarianTrueBeam®STx上长时间观察到的每日输出变化高达±2％；这些输出变化被假定为密封的监控室与大气连通的结果。使用TrueBeam®上的电离室阵列（DQA3）和非晶硅平板检测器（IDU）测量的相对于基线的每日输出变化与每日温度-压力校正（P _TP）由DQA3中的传感器确定。使用Farmer®电离室进行了5个小时的输出测量，在此期间，监控室的温度受到控制。在所有测量中，使用DQA3和IDU测量的相对于基线的输出变化百分比之间的均方根差为0.50％。在对输出量和P _TP的每日变化进行的7个月回顾性研究中，在前5个月中， 观察到输出量与P _TP之间的相关性较弱（R ² = 0.30）。在最后两个月，每日产量变化与P _TP的变化呈线性相关，斜率为0.84（R ² = 0.89）。在监控室受控加热和冷却期间针对环境温度和压力校正的电离测量值与密封监控室的预期值相差最多4.6％，但与不确定范围内的空气流通监控室的预期值相符（1.3％，k  = 2）。更换减压监测室之后，输出的每日百分比变化与P _TP之间没有相关性（R ²  = 0.09）。演示了控制图的实用性，可以更早地识别出密封的监控室灵敏度的变化。