质子放疗中CT值到相对阻止本领转换关系依赖因素研究及其在脂肪区的改进

Study on the conversion between CT hounsfield units and relative stopping power in proton therapy and the improvement in adipose tissue

目的介绍一种质子放疗过程中由CT值(HU)转换到相对阻止本领(RSP)的校准方法,提高其在脂肪区间的校准精度。方法采用化学计量校准方法,计算人体组织的HU和RSP。利用动物组织模拟患者皮下脂肪组织,并测量该动物组织的HU和RSP。通过分段拟合HU和RSP,分析皮下脂肪组织对HU-RSP转换关系的影响。结果计入皮下脂肪测量值,可显著提高HU-RSP转换曲线在脂肪区间的转换精度。不同平均电离能数值的选择对HU-RSP转换关系的影响<0.6%,质子束入射能量的差异对HU-RSP的影响<0.8%。结论皮下脂肪的计入提高了HU-RSP转换关系在脂肪区间内的精度,降低了质子在脂肪区间的射程误差。

Objective To introduce a method for calibrating the conversion from CT Hounsfield units (HU) to relative stopping power (RSP) for proton therapy, and improve the precision of the conversion in the region for adipose tissues. Methods The HU and RSP values of human tissues were calculated by a stoichiometric calibration method. Animal tissue was used to simulate subcutaneous adipose tissue of patients, and the HU and RSP of the animal tissue were measured. The effect of subcutaneous adipose tissue on conversion between HU and RSP were analyzed by piecewise fitting. Results The precision of conversion curve was improved significantly with the measured HU and RSP of adipose tissue in the fitting. The effect caused by different choice in different ionization energy was less than 0.6%, and the effect of proton energy differential was less than 0.8%. Conclusions The precision of conversion curve for the transformation of HU into RSP in adipose tissues could be improved by taking subcutaneous adipose tissue into account, which would reduce the range error of proton beams when such tissues are present in the target volumes or in the beam path.