Abstract：
The dosimetric output of a 6FFF beam, produced from a Varian TrueBeam linac exhibited an unexpected downward trend over time that was contrary to well-established expectations. To elucidate the cause of this uncharacteristic trend, a review of the linac\'s quality control results over its lifetime was performed, including, constancy checks of the dosimetric output, beam energy, flatness and symmetry, and percentage depth dose characteristics. These results were supplemented with a comprehensive series of measurements including flatness and symmetry measurements with a 1D-diode array, high-resolution measurements of the photon beam\'s build-up region with a parallel-plate chamber and measurement of the beam\'s output as a function of the x-ray target position. The review of the linac\'s QC results and supplemental tests identified no deviations in the linac\'s performance from its commissioning and baseline measurements. However, the 6FFF beam output exhibited a significant dependence on the target location relative to its default position, increasing by 5.43 % with a 0.5 mm target translation, indicating that target degradation was the cause of the atypical output trend. The change in output behaviour was believed to be the result of primary electrons escaping the degraded target and interacting with the linac\'s monitor chamber. Replacement of the x-ray target caused the 6FFF output to realign with expected trends. Target degradation was uncovered due to a robust quality control trending database and awareness of typical output behaviour. These results demonstrate the importance of data trending to identify component failure and provide centres with knowledge to recognise this potential fault.
摘要：
6FFF光束的剂量测定输出，VarianTrueBeam直线加速器生产的产品随着时间的推移表现出意想不到的下降趋势，这与既定的预期相反。为了阐明这种不典型趋势的原因，对直线加速器在其生命周期内的质量控制结果进行了审查，包括,剂量测定输出的恒定性检查，光束能量,平整度和对称性，和百分比深度剂量特征。这些结果补充了一系列全面的测量，包括使用一维二极管阵列进行的平面度和对称性测量，具有平行板室的光子束的建立区域的高分辨率测量和作为X射线目标位置的函数的束输出的测量。对直线加速器的质量控制结果和补充测试的审查发现直线加速器的性能与调试和基线测量没有偏差。然而,6FFF光束输出表现出对目标位置相对于其默认位置的显著依赖性，以0.5毫米的目标平移增加5.43%，表明目标退化是非典型输出趋势的原因。输出行为的变化被认为是初级电子逃离退化目标并与直线加速器的监视器室相互作用的结果。更换X射线靶导致6FFF输出与预期趋势重新对准。由于强大的质量控制趋势数据库和对典型输出行为的认识，未发现目标退化。这些结果证明了数据趋势对识别组件故障的重要性，并为中心提供了识别此潜在故障的知识。
