质子点扫描照射位置控制系统设计

Design of a proton spot scanning position control system

上海质子治疗装置的束配系统采用点扫描的治疗模式。为了满足照射率与照射剂量分布精度的要求,需要设计一套精确的点扫描照射位置控制系统。点扫描照射位置控制系统通过二次多项式以及薄板样条曲线拟合得到了扫描磁场强度与位置电离室读出、扫描磁场强度与等中心束流位置之间的映射关系。在实际治疗时,通过治疗计划系统要求的等中心束流位置计算出目标扫描磁场强度,采用前馈算法控制扫描磁场到达目标磁场,同时通过比较位置电离室读出与目标位置之间的偏差来校正下一个点的照射位置。带束测试结果显示:在等中心处测得的质子束流在X方向和Y方向的照射位置偏差的最大值分别为0.8 mm和0.6 mm,均方值均为0.2 mm。该照射位置控制系统能够实现精确的点扫描照射。

[Background] Spot scanning is adopted as the operating mode for the beam delivery system of the Shanghai Proton Therapy Facility. [Purpose] To meet the needs of dose rate and dose accuracy, a precise spot scanning position control system was developed. [Methods] Quadratic polynomial and smoothed thin-plate splines fitting has been adopted in mapping the relationship between the target scanning magnet strength and spot position at ISO center, and the relationship between the target scanning magnet strength and the spot position at ion chamber. During the treatment process, the target magnetic field is calculated according to the target spot position at ISO center given by the treatment planning system. A feed forward magnet strength control strategy is used in controlling the scanning magnetic to reach the target magnetic field. The position of the following spot is corrected through comparing the target spot position and the spot position readout from the ion chamber. [Results] Beam test showed that the maximum position error in X and Y direction was 0.8 mm and 0.6 mm respectively, and both of the root-mean-square were 0.2 mm. [Conclusion] Accurate spot scanning has been achieved by the spot scanning position control system.