调强适形放射治疗剂量验证中CT密度值的校准分析

Calibration of CT density values in dosimetry verification of intensity modulated radiation therapy

目的基于调强适形放射治疗（IMRT）验证体模，分析放射治疗计划系统中CT密度转换曲线在IMRT剂量验证中的影响，并加以校准。方法利用CT电子密度体模刻度计划系统中的CT密度转换曲线，取12例鼻咽癌患者的IMRT计划，分别移植至IMRT验证体模，计算剂量分布，并于剂量均匀处放置测量点，利用电离室测量出其实际剂量。将IMRT验证体模的物理密度与对应的CT值输入计划系统，对CT密度转换曲线进行修正，重新计算剂量，在保持其他计划参数一致的前提下，比较前后两次计算值与实测值的差异。结果12例鼻咽癌IMRT计划中，前后两次计算值和实测值的平均误差分别为1．96％±0．87％和0．63％±0．74％，修正后的计算值误差控制在±2％以内，而修正前计算值的最大误差为3．24％。结论利用修正后的CT密度转换曲线，计划中的剂量计算值更加贴近实际测量值。为提高IMRT剂量验证的精确性，验证体模在使用之前必须进行CT密度值的校准、确认。

Objective Based on intensity modulated radiation therapy （IMRT） phantom, the impact of CT-to-density conversion curve on dosimetry verification of IMRT is investigated and calibrated. Methods The electron density phantom was used to establish the CT-to-density conversion curve in radiation treatment planning system. IMRT plans of 12 nasopharynx carcinoma patients were chosen, copied to IMRT phantom and computed for the dose distribution. For each plan a measured point was put at the place where the dose was well-distributed and its dose value was measured using the ionization chamber. The physical density of IMRT phantom and its CTvalue were input into the planning system, to make a calibration for the CT-to-density conversion curve. The dose distribution was recomputed for each IMRT plan. Other parameters were kept the same in the plans and the differences between the computed dose values before and after correction were compared with the measured values. Results In 12 nasopharynx carcinoma IMRT plans, the average error of computed dose values was 1.96% ± 0.87% before correction and 0.63%± 0.74% after correction, compared with measured values. The error between measured values and computed values after correction was less than ±2% whereas the maximum error of computed values before correction was 3.24%. Conclusions The computed dose values are closer to the measured values when using the calibrated CT-to-density conversion curve. The CT density values of IMRT phantom should be verified before usage, so as to increase the accuracy of IMRT dosimetry verification.