Abstract：
A fundamental parameter to evaluate the beam delivery precision and stability on a clinical linear accelerator (linac) is the focal spot position (FSP) measured relative to the collimator axis of the radiation head. The aims of this work were to evaluate comprehensive data on FSP acquired on linacs in clinical use and to establish the ability of alternative phantoms to detect effects on patient plan delivery related to FSP. FSP measurements were conducted using a rigid phantom holding two ball-bearings at two different distances from the radiation source. Images of these ball-bearings were acquired using the electronic portal imaging device (EPID) integrated with each linac. Machine QA was assessed using a radiation head-mounted PTW STARCHECK phantom. Patient plan QA was investigated using the SNC ArcCHECK phantom positioned on the treatment couch, irradiated with VMAT plans across a complete 360° gantry rotation and three X-ray energies. This study covered eight Elekta linacs, including those with 6 MV, 18 MV, and 6 MV flattening-filter-free (FFF) beams. The largest range in the FSP was found for 6 MV FFF. The FSP of one linac, retrofitted with 6 MV FFF, displayed substantial differences in FSP compared to 6 MV FFF beams on other linacs, which all had FSP ranges less than 0.50 mm and 0.25 mm in the lateral and longitudinal directions, respectively. The PTW STARCHECK phantom proved effective in characterising the FSP, while the SNC ArcCHECK measurements could not discern FSP-related features. Minor variations in FSP may be attributed to adjustments in linac parameters, component replacements necessary for beam delivery, and the wear and tear of various linac components, including the magnetron and gun filament. Consideration should be given to the ability of any particular phantom to detect a subsequent impact on the accuracy of patient plan delivery.
摘要：
评估临床直线加速器（直线加速器）上的光束传递精度和稳定性的基本参数是相对于辐射头的准直器轴线测量的焦斑位置（FSP）。这项工作的目的是评估在临床使用中在直线加速器上获得的FSP的综合数据，并建立替代体模的能力，以检测与FSP相关的患者计划交付的影响。使用刚性体模将两个滚珠轴承保持在距辐射源的两个不同距离处进行FSP测量。这些滚珠轴承的图像是使用与每个直线加速器集成的电子射野成像设备（EPID）获取的。使用放射头安装的PTWSTARCHECK体模评估机器QA。使用位于治疗床上的SNCArcCHECK体模研究患者计划QA,用VMAT计划在完整的360°机架旋转和三个X射线能量中照射。这项研究涵盖了八个Elekta直线赛车，包括那些有6MV的，18MV,和6个MV无平坦滤波（FFF）光束。FSP中的最大范围是6MVFFF。一个直线加速器的FSP，加装6MVFFF，与其他直线加速器上的6MVFFF梁相比，FSP显示出实质性差异，在横向和纵向上的FSP范围均小于0.50mm和0.25mm，分别。PTWSTARCHECK体模在表征FSP方面被证明是有效的，而SNCArcCHECK测量无法辨别FSP相关特征。FSP的微小变化可能归因于直线加速器参数的调整，梁交付所需的部件更换，以及各种直线加速器部件的磨损，包括磁控管和枪灯丝。应考虑任何特定体模检测对患者计划交付的准确性的后续影响的能力。
