Monaco治疗计划系统中量化并构建电离室有效点计算模型

Quantification and construction of the effective point calculation model of ionization chamber in Monacao treatment planning system

目的蒙卡剂量计算中的统计噪声会影响有效点剂量测量的精度。采用用户定义的球体积替代有效点，围绕有效点进行球体取样可降低随机误差提高剂量统计精度。方法将0．125cm。电离室（IC）放置在圆柱型均匀模体的中心分别在0。和90。进行直接剂量测量。在扫描的cT体模系列中，勾画Ic敏感体积长度，将等中心点定义为模拟有效点。根据测量模式在治疗计划系统中模拟所有射野，采用2 mm体素计算网格间距要求相对标准偏差≤0．5％。在不同的采样球半径（2．5、2．0、1．5、1．0 mm）下对3种不同电子密度（ED）IC模型（模型A食管腔电子密度0．210 g,／cm^3、模型B空气电子密度0．001 g／cm^3和模型C默认cT电子密度）计算值与测量值进行比较，确定MC计算统计不确定度对剂量精度影响。结果在Monaco计划系统中对Ic使用模型A且取样球半径为1．5 mm时，计算统计值与测量值的绝对平均偏差最小为0．49％。当IC使用模型B和模型c时。推荐统计采样球半径为2．5 mm，绝对平均偏差分别为0．61％和0．70％。结论在Monaco治疗计划中，对31010电离室的有效点测量模型推荐使用电子密度为0．210 g／cm^3和取样半径为1．5 mm球体积替代有效点剂量测量以减小蒙卡随机统计误差。

[ Abstract] Objective Because of statistical noise in Monte Carlo dose calculations, the effective point doses may not be accurately calculated. A user-defined sphere volume was adopted to substitute the effective point to take sphere sampling around the effective point, which minimize the random errors and improve the accuracy of statistical dose. Methods Direct dose measurements were performed at 0~ and 90~ using a 0. 125 cm^3 Semiflex ionization chamber （IC） 31010 isocentrically placed in the center of a homogeneous Cylindric sliced RW3 phantom （PTW, Germany）. In the scanned CT phantom series, the sensitive volume length of the IC （6. 5 ram） was delineated and the isocenter was defined as the simulated effective point. All beams were simulated in the treatment planning system （TPS） in accordance to the measured model. The grid spacing was calculated by 2 mm voxels and the relative standard deviation should be ~〈 0. 5%. The statistical and measured doses were statistically compared among three IC models with different electron densities （ ED ; esophageal lumen ED = 0. 210 g/cm^3 for model A, air ED = 0. 001 g/cm^3 for model B and the default CT scanned ED for model C） at different sampling sphere radius （ 2. 5,2. 0,1.5 and I. 0 ram） to evaluate the effect of Monte Carlo.calculation uncertainty upon the dose accuracy. Results In the Monaco TPS, the statistical value was in the highest accordance with the measured value with an absolute average deviation of 0. 49% when the IC was set as esophageal lumen ED = 0. 210 g/cm^3 and the sampling sphere radius was 1.5 mm. When the IC was set as air ED=0. 001 g/cm^3 and default CT scanned ED,and, the recommended statistical sampling sphere radius was 2. 5 mm, the absolute average deviations were 0. 61% and O. 70%. Conclusion In the Monaco TPS, the calculation model with an ED of 0. 210 g/cm^3 and a sampling radius of 1.5 mm is recommended for the ionization chamber 31010 to substitute the effective point dose measurement to decrease the random stochastic errors of Monte Carlo.