Quantification and construction of the effective point calculation model of ionization chamber in Monacao treatment planning system
10.3760/cma.j.issn.1004-4221.2018.10.009
- VernacularTitle:Monaco治疗计划系统中量化并构建电离室有效点计算模型
- Author:
Ruohui ZHANG
1
,
2
;
Wenwen BAI
;
Yulan GAO
;
Mingchang MIAO
;
Shiguang WANG
;
Yuanming FENG
;
Zifeng CHI
Author Information
1. 300072 天津大学精密仪器与光电子工程学院生物医学工程系
2. 050011 石家庄,河北医科大学第四医院放疗科
- Keywords:
Monte Carlo;
Ionization chamber model;
Treatment planning system;
Sphere sampling
- From:
Chinese Journal of Radiation Oncology
2018;27(10):916-919
- CountryChina
- Language:Chinese
-
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 cm3 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 mm) 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/cm3 for model A,air ED =0.001 g/cm3 for model B and the default CT scanned ED for model C) at different sampling sphere radius (2.5,2.0,1.5 and 1.0 mm) 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/cm3 and the sampling sphere radius was 1.5 mm.When the IC was set as air ED=0.001 g/cm3 and default CT scanned ED,and,the recommended statistical sampling sphere radius was 2.5 mm,the absolute average deviations were 0.61％ and 0.70％.Conclusion In the Monaco TPS,the calculation model with an ED of 0.210 g/cm3 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.
