Chua Clifford Ghee Ann, Furutani Keith M, Lee Kang Hao, Lew Kah Seng, Koh Calvin Wei Yang, Wibawa Andrew, Master Zubin, Lee James Cheow Lei, Beltran Chris J, Park Sung Yong, Tan Hong Qi
Division of Radiation Oncology, National Cancer Centre Singapore, Singapore, Singapore.
Division of Physics and Applied Physics, School of Physical and Mathematical Science, Nanyang Technological University, Singapore, Singapore.
Med Phys. 2025 Aug;52(8):e18050. doi: 10.1002/mp.18050.
Dose-driven continuous scanning (DDCS) is a type of beam delivery in Hitachi pencil beam scanning (PBS) particle therapy system. Particle beam is irradiated continuously between spots to achieve a higher effective dose rate and a shorter beam delivery time. The beam is only turned off at break spots, the end of a treatment layer and during spill change. The break spot is a unique feature of the DDCS introduced by the treatment planning system (TPS) to interrupt continuous scanning. DDCS separates the dose delivery of a spot into a move dose and stop dose components. The move dose is controlled by the scanning speed and beam current, while the stop dose delivers the remaining monitor units (MUs). Hence, rigorous quality assurance (QA) of the scanning speed and functionality of break spot are required to ensure safe and successful DDCS delivery.
We aim to report for the first time the QA methodology and result for DDCS, focusing on the scanning speed and break spot validation.
A rectangular spiral irradiation pattern with different degree of break spots were irradiated on a 2D strip ionization chamber (CROSSmini) detector using 50 time resolution to validate the break spots. Four methods of measuring scanning speeds were proposed. Using a spiral irradiation pattern, methods one and two applied linear regression to the scanning time measured by the CROSSmini for different inter-spot distances, with measurements taken at sampling rates of 5 and 20 kHz, respectively. Methods three and four measured the scanning time of single inter-spot distances in the X and Y directions. Method three measured the scanning time with the CROSSmini while method four measured the rise and fall time of the scanning magnet (SCM) current. All measurements were performed with 70.2, 150.2 and 228.7 MeV proton beam energies and three different beam currents of 8, 14 and 20 MU/s. Lastly, the scanning speed tolerance limit was calculated analytically under the requirement of avoiding a beam abort when the stop dose was less than zero.
The data recorded by the CROSSmini under the spiral irradiation patterns with break spots was reconstructed into a 2D dose, and the break spots' operations were visually validated for all the energies and beam currents. There were no statistically significant difference in the measured X and Y scanning speeds across the four methods. However, the last method involving the SCM current yielded the largest type A uncertainty of up to 25% ( ) due to the noisy signal. The first method with the linear regression and 5 kHz acquisition rate was preferred due to a smaller type A uncertainty of up to 2.0% and a lower false negative rate in detecting the rising and falling edges during scanning. The calculated tolerance limit based on our current DDCS setting was found to be 0.50 times of the expected scanning speed.
We have introduced QA protocols for DDCS, designed to ensure complete and safe beam delivery without interruptions caused by move doses exceeding planned spot doses.
剂量驱动连续扫描（DDCS）是日立笔形束扫描（PBS）粒子治疗系统中的一种束流输送方式。粒子束在各点之间连续照射，以实现更高的有效剂量率和更短的束流输送时间。束流仅在断点、治疗层末端和剂量溢出变化期间关闭。断点是治疗计划系统（TPS）引入的DDCS的一个独特特征，用于中断连续扫描。DDCS将一个点的剂量输送分为移动剂量和停止剂量分量。移动剂量由扫描速度和束流控制，而停止剂量输送剩余的监测单位（MU）。因此，需要对扫描速度和断点功能进行严格的质量保证（QA），以确保安全、成功地进行DDCS输送。
我们旨在首次报告DDCS的QA方法和结果，重点是扫描速度和断点验证。
使用二维条带电离室（CROSSmini）探测器，以50 时间分辨率照射具有不同断点程度的矩形螺旋照射模式，以验证断点。提出了四种测量扫描速度的方法。使用螺旋照射模式，方法一和方法二对CROSSmini在不同点间距下测量的扫描时间应用线性回归，采样率分别为5 kHz和20 kHz。方法三和方法四测量X和Y方向上单个点间距的扫描时间。方法三使用CROSSmini测量扫描时间，而方法四测量扫描磁铁（SCM）电流的上升和下降时间。所有测量均在70.2、150.2和228.7 MeV质子束能量以及8、14和20 MU/s三种不同束流下进行。最后，在避免停止剂量小于零时束流中断的要求下，通过解析计算扫描速度公差极限。
将CROSSmini在带有断点的螺旋照射模式下记录的数据重建为二维剂量，并对所有能量和束流下断点的操作进行了直观验证。四种方法测量的X和Y扫描速度在统计学上没有显著差异。然而，由于信号噪声，涉及SCM电流的最后一种方法产生的A类不确定度最大，高达25%（ ）。由于A类不确定度较小，高达2.0% ，且在扫描期间检测上升和下降边缘时假阴性率较低，因此首选采用线性回归和5 kHz采集率的第一种方法。基于我们当前DDCS设置计算的公差极限为预期扫描速度的0.50倍。
我们引入了DDCS的QA协议，旨在确保完整、安全的束流输送，且不会因移动剂量超过计划点剂量而中断。
