调强放射治疗剂量学中的不确定度。

Uncertainties in IMRT dosimetry.

机构信息

Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111, USA.

出版信息

Med Phys. 2010 Jun;37(6):2491-500. doi: 10.1118/1.3413997.

DOI:10.1118/1.3413997

PMID:20632560

Abstract

PURPOSE

The purpose of this study is to investigate some characteristics of the beam delivery system and their effects on the dose distribution of intensity-modulated radiation therapy (IMRT) and the results of patient-specific IMRT quality assurance (QA). These characteristics include the accelerator source distribution and multileaf collimator (MLC) geometry.

METHODS

Monte Carlo dose calculations based on intensity maps that were built from the actual deliverable IMRT leaf sequences with and without considering the characteristics of the beam delivery system were performed in this study using in-house Monte Carlo software. The effect of the resolution of the intensity maps on the dose distribution was investigated first. The mean dose of the treatment target and the voxel doses in the high dose region of seven IMRT plans generated by different treatment planning systems for Varian 21EX and Siemens Primus linear accelerators were used for comparison and evaluation.

RESULTS

The results show that a 0.2 x 0.2 mm2 or smaller pixel size is needed for the intensity maps for accurate IMRT dose calculation. The extrafocal source, MLC leaf thickness, leakage, tongue-and-groove structure, and the effective leaf offset can affect the mean dose by up to 1.5%, 4.5%, 5.6%, 5.3%, and 7.8%, respectively, when these factors are considered separately. They also cause significant uncertainties to the voxel dose with standard deviations up to 2.5%, 0.7%, 2.1%, 1.3%, and 5%, respectively. The overall effect on the mean dose is up to 8% and the standard deviation of the voxel dose uncertainty is up to 6.4% when all the effects are included. The maximum standard deviation is reduced to 4.6% if the voxel size of the dose calculation matrix is increased from 0.04 to 0.3 cm3 to make it comparable with the sensitive volume of the ionization chamber used for IMRT QA measurements.

CONCLUSIONS

It can be concluded that the characteristics of the beam delivery system are the major contributors to the uncertainty of measurement-based IMRT QA because most of them are not fully considered in the currently available treatment planning systems.

摘要

目的

本研究旨在探讨束流传输系统的一些特性及其对强度调制放射治疗（IMRT）剂量分布和个体化 IMRT 质量保证（QA）结果的影响。这些特性包括加速器源分布和多叶准直器（MLC）几何形状。

方法

本研究使用内部蒙特卡罗软件，根据实际可交付的 IMRT 叶片序列构建的强度图进行蒙特卡罗剂量计算，同时考虑和不考虑束流传输系统的特性。首先研究了强度图的分辨率对剂量分布的影响。比较和评估了不同治疗计划系统为瓦里安 21EX 和西门子 Primus 直线加速器生成的七个 IMRT 计划的靶区平均剂量和高剂量区的体素剂量。

结果

结果表明，对于准确的 IMRT 剂量计算，强度图需要 0.2 x 0.2 毫米或更小的像素尺寸。当分别考虑外焦点源、MLC 叶片厚度、漏射、齿槽结构和有效叶片偏移时，它们分别会使平均剂量产生高达 1.5%、4.5%、5.6%、5.3%和 7.8%的影响。它们还会导致体素剂量产生显著的不确定性，标准偏差高达 2.5%、0.7%、2.1%、1.3%和 5%。当考虑所有这些影响时，对平均剂量的总体影响高达 8%，体素剂量不确定性的标准偏差高达 6.4%。如果将剂量计算矩阵的体素尺寸从 0.04 厘米增加到 0.3 厘米，使其与用于 IMRT QA 测量的电离室的灵敏体积相匹配，则最大标准偏差可降低至 4.6%。