120 叶多叶准直器的蒙特卡罗实现、验证和特性描述。

Monte Carlo implementation, validation, and characterization of a 120 leaf MLC.

机构信息

Division of Medical Radiation Physics, Inselspital and University of Bern, CH-3010 Bern, Switzerland.

出版信息

Med Phys. 2011 Oct;38(10):5311-20. doi: 10.1118/1.3626485.

DOI:10.1118/1.3626485

PMID:21992349

Abstract

PURPOSE

Recently, the new high definition multileaf collimator (HD120 MLC) was commercialized by Varian Medical Systems providing high resolution in the center section of the treatment field. The aim of this work is to investigate the characteristics of the HD120 MLC using Monte Carlo (MC) methods.

METHODS

Based on the information of the manufacturer, the HD120 MLC was implemented into the already existing Swiss MC Plan (SMCP). The implementation has been configured by adjusting the physical density and the air gap between adjacent leaves in order to match transmission profile measurements for 6 and 15 MV beams of a Novalis TX. These measurements have been performed in water using gafchromic films and an ionization chamber at an SSD of 95 cm and a depth of 5 cm. The implementation was validated by comparing diamond measured and calculated penumbra values (80%-20%) for different field sizes and water depths. Additionally, measured and calculated dose distributions for a head and neck IMRT case using the DELTA(4) phantom have been compared. The validated HD120 MLC implementation has been used for its physical characterization. For this purpose, phase space (PS) files have been generated below the fully closed multileaf collimator (MLC) of a 40 × 22 cm(2) field size for 6 and 15 MV. The PS files have been analyzed in terms of energy spectra, mean energy, fluence, and energy fluence in the direction perpendicular to the MLC leaves and have been compared with the corresponding data using the well established Varian 80 leaf (MLC80) and Millennium M120 (M120 MLC) MLCs. Additionally, the impact of the tongue and groove design of the MLCs on dose has been characterized.

RESULTS

Calculated transmission values for the HD120 MLC are 1.25% and 1.34% in the central part of the field for the 6 and 15 MV beam, respectively. The corresponding ionization chamber measurements result in a transmission of 1.20% and 1.35%. Good agreement has been found for the comparison between transmission profiles resulting from MC simulations and film measurements. The simulated and measured values for the penumbra agreed within <0.5 mm for all field sizes, depths, and beam energies, and a good agreement has been found between the measured and the calculated dose distributions for the IMRT case. The total energy spectra are almost identical for the three MLCs. However, the mean energy, fluence and energy fluence are significantly different. Due to the different leaf widths of the MLCs, the shape of these distributions is different, each representing its leave structure. Due to the increase in width from the inner to the outer HD120 MLC leaves, the fluence and energy fluence clearly decrease below the outer leaves. The MLC80 and the M120 MLC resulted in an increase of the fluence and energy fluence compared with those resulted for the HD120 MLC. The dose reduction can exceed 20% compared with the dose of the open field due to the tongue and groove design of the HD120 MLC.

CONCLUSIONS

The HD120 MLC has been successfully implemented into the SMCP. Comparisons between MC calculations and measurements show very good agreement. The SMCP is now able to calculate accurate dose distributions for treatment plans using the HD120 MLC.

摘要

目的

最近，瓦里安医疗系统（Varian Medical Systems）推出了新型高清多叶准直器（HD120 MLC），可在治疗野中心区域提供高分辨率。本研究旨在利用蒙特卡罗（MC）方法研究 HD120 MLC 的特性。

方法

根据制造商的信息，将 HD120 MLC 集成到现有的瑞士 MC 计划（SMCP）中。通过调整物理密度和相邻叶片之间的气隙来实现该集成，以匹配 Novalis TX 的 6 和 15 MV 光束的传输特性测量。这些测量是在水水箱中使用 Gafchromic 胶片和电离室在 SSD 为 95 cm 和水深为 5 cm 的情况下进行的。通过比较不同射野大小和水深的钻石测量和计算的半影值（80%-20%）来验证实现。此外，还比较了使用 DELTA(4)模体的头颈部调强放疗（IMRT）病例的测量和计算剂量分布。使用经过验证的 HD120 MLC 实现来进行物理特性描述。为此，为 40×22 cm(2)射野大小的完全关闭多叶准直器（MLC）生成了下方的相空间（PS）文件，用于 6 和 15 MV。分析了 PS 文件的能量谱、平均能量、注量和垂直于 MLC 叶片方向的能量注量，并将其与使用成熟的瓦里安 80 叶（MLC80）和千禧年 M120（M120 MLC）MLC 的相应数据进行了比较。此外，还描述了 MLC 齿槽设计对剂量的影响。

结果

HD120 MLC 在 6 和 15 MV 光束的射野中心部分的计算传输值分别为 1.25%和 1.34%。相应的电离室测量结果为 1.20%和 1.35%。MC 模拟产生的传输特性与胶片测量结果之间的比较吻合良好。模拟和测量的半影值在所有射野大小、深度和射束能量下均相差<0.5 mm，并且在 IMRT 病例中，测量和计算的剂量分布之间存在良好的一致性。总能量谱对于三个 MLC 几乎相同。然而，平均能量、注量和能量注量则有显著差异。由于 MLC 的叶片宽度不同，这些分布的形状也不同，每个分布都代表其叶片结构。由于 HD120 MLC 从内到外叶片宽度的增加，注量和能量注量在外侧叶片下方明显下降。与 HD120 MLC 相比，MLC80 和 M120 MLC 导致注量和能量注量增加。由于 HD120 MLC 的齿槽设计，与开放野相比，剂量减少可超过 20%。