Beam position uncertainties along the beam trajectory arise from the accelerator, beamline, and scanning magnets (SMs). They can be monitored in real time, e.g., through strip ionization chambers (ICs), and treatments can be paused if needed. Delivery is more reliable and accurate if the beam position is projected from monitored nozzle parameters to the isocenter, allowing for accurate online corrections to be performed. Beam position projection algorithms are also used in post-delivery log file analyses. In this paper, we investigate the four potential algorithms that can be applied to all pencil beam scanning (PBS) nozzles. For some combinations of nozzle configurations and algorithms, however, the projection uses beam properties determined offline (e.g., through beam tuning or technical commissioning). The best algorithm minimizes either the total uncertainty (i.e., offline and online) or the total offline uncertainty in the projection. Four beam position algorithms are analyzed (A1–A4). Two nozzle lengths are used as examples: a large nozzle (1.5 m length) and a small nozzle (0.4 m length). Three nozzle configurations are considered: IC after SM, IC before SM, and ICs on both sides. Default uncertainties are selected for ion chamber measurements, nozzle entrance beam position and angle, and scanning magnet angle. The results for other uncertainties can be determined by scaling these results or repeating the error propagation. We show the propagation of errors from two locations and the SM angle to the isocenter for all the algorithms. The best choice of algorithm depends on the nozzle length and is A1 and A3 for the large and small nozzles, respectively. If the total offline uncertainty is to be minimized (a better choice if the offline uncertainty is not stable), the best choice of algorithm changes to A1 for the small nozzle for some hardware configurations. Reducing the nozzle length can help to reduce the gantry size and make proton therapy more accessible. This work is important for designing smaller nozzles and, consequently, smaller gantries. This work is also important for log file analyses.
沿束流轨迹的束流位置不确定性源于加速器、束流传输线及扫描磁铁。这些不确定性可通过如条带电离室等设备实时监测，并在必要时暂停治疗。若将束流位置从监测的喷嘴参数投影至等中心点，可实现精确的在线校正，从而提升治疗实施的可靠性与准确性。束流位置投影算法同样适用于治疗后的日志文件分析。本文研究了可应用于所有笔形束扫描喷嘴的四种潜在算法。然而，对于某些喷嘴配置与算法的组合，投影过程需使用离线确定的束流特性（如通过束流调谐或技术调试）。最优算法应使投影中的总不确定性（即离线和在线不确定性之和）或总离线不确定性最小化。本文分析了四种束流位置算法（A1–A4），并以长喷嘴（1.5米）和短喷嘴（0.4米）两种长度为例，考察了三种喷嘴配置：扫描磁铁后置电离室、扫描磁铁前置电离室以及两侧均配置电离室。研究为电离室测量、喷嘴入口束流位置与角度以及扫描磁铁角度设定了默认不确定度，其他不确定度的影响可通过比例缩放或重复误差传递分析确定。我们展示了所有算法中从两个位置及扫描磁铁角度到等中心点的误差传递过程。算法的最佳选择取决于喷嘴长度：长喷嘴适用A1算法，短喷嘴适用A3算法。若以最小化总离线不确定性为目标（在离线不确定性不稳定的情况下更为适宜），对于某些硬件配置，短喷嘴的最佳算法将变为A1。缩短喷嘴长度有助于缩小机架尺寸，提升质子治疗的可及性。本研究对设计更紧凑的喷嘴及机架具有重要意义，同时对日志文件分析也具有重要参考价值。
Beam Position Projection Algorithms in Proton Pencil Beam Scanning
肿瘤(癌症)患者之家