基于模型的专用 6 MV 立体定向外科单元的锥形束离轴比计算。

Model-based calculations of off-axis ratio of conic beams for a dedicated 6 MV radiosurgery unit.

机构信息

Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030, USA.

出版信息

Med Phys. 2010 Oct;37(10):5442-7. doi: 10.1118/1.3488900.

DOI:10.1118/1.3488900

PMID:21089780

Abstract

PURPOSE

Because the small-radius photon beams shaped by cones in stereotactic radiosurgery (SRS) lack lateral electronic equilibrium and a detector's finite cross section, direct experimental measurement of dosimetric data for these beams can be subject to large uncertainties. As the dose calculation accuracy of a treatment planning system largely depends on how well the dosimetric data are measured during the machine's commissioning, there is a critical need for an independent method to validate measured results. Therefore, the authors studied the model-based calculation as an approach to validate measured off-axis ratios (OARs).

METHODS

The authors previously used a two-component analytical model to calculate central axis dose and associated dosimetric data (e.g., scatter factors and tissue-maximum ratio) in a water phantom and found excellent agreement between the calculated and the measured central axis doses for small 6 MV SRS conic beams. The model was based on that of Nizin and Mooij ["An approximation of central-axis absorbed dose in narrow photon beams," Med. Phys. 24, 1775-1780 (1997)] but was extended to account for apparent attenuation, spectral differences between broad and narrow beams, and the need for stricter scatter dose calculations for clinical beams. In this study, the authors applied Clarkson integration to this model to calculate OARs for conic beams. OARs were calculated for selected cones with radii from 0.2 to 1.0 cm. To allow comparisons, the authors also directly measured OARs using stereotactic diode (SFD), microchamber, and film dosimetry techniques. The calculated results were machine-specific and independent of direct measurement data for these beams.

RESULTS

For these conic beams, the calculated OARs were in excellent agreement with the data measured using an SFD. The discrepancies in radii and in 80%-20% penumbra were within 0.01 cm, respectively. Using SFD-measured OARs as the reference data, the authors found that the calculated OARs were more accurate than those measured with a microchamber or film dosimetry.

CONCLUSIONS

The model produces sufficiently accurate conic beam dosimetric data that can be used to validate direct measurement results for such beams.

摘要

目的

由于立体定向放射外科（SRS）中使用的锥形小射束缺乏侧向电子平衡和探测器的有限横截面，因此这些射束的剂量学数据的直接实验测量可能会存在很大的不确定性。由于治疗计划系统的剂量计算准确性在很大程度上取决于机器调试期间剂量学数据的测量情况，因此迫切需要一种独立的方法来验证测量结果。因此，作者研究了基于模型的计算方法，以验证测量的离轴比（OAR）。

方法

作者之前使用两分量分析模型在水模体中计算了小 6 MV SRS 锥形束的中心轴剂量和相关剂量学数据（例如散射因子和组织最大比），并发现计算和测量的中心轴剂量之间具有极好的一致性。该模型基于 Nizin 和 Mooij 的模型[“窄光子束中心轴吸收剂量的近似值”，《医学物理》24，1775-1780（1997）]，但进行了扩展，以考虑到明显衰减，宽束和窄束之间的光谱差异以及对临床束更严格的散射剂量计算的需求。在这项研究中，作者将 Clarkson 积分应用于该模型以计算锥形束的 OAR。计算了半径从 0.2 到 1.0 cm 的选定锥形的 OAR。为了进行比较，作者还使用立体定向二极管（SFD），微室和胶片剂量测量技术直接测量 OAR。计算结果是特定于机器的，与这些射束的直接测量数据无关。