Das I J, Cheng C W, Healey G A
Department of Radiation Oncology, University of Massachusetts Medical Center, Worcester.
Int J Radiat Oncol Biol Phys. 1995 Jan 1;31(1):157-63. doi: 10.1016/0360-3016(94)E0299-Y.
A method is provided for the optimum field size and the choice of isodose line for the dose prescription in electron beam therapy.
Electron beam dose uniformity was defined in terms of target coverage factor (TCF) which is an index of dose coverage of a given treatment volume. The TCF was studied with respect to the field size, the beam energy, and the isodose level for prescription from the measured data for various accelerators. The effect of the TCF on air gap between electron applicator/cone and the surface was investigated. Electron beams from scattering foil and scanned beam units were analyzed for the target coverage.
A mathematical method is provided to optimize a field size for target coverage by a given isodose line in terms of TCF which is strongly dependent on the type of accelerator and the design of the collimator. For a given type of collimating system, the TCF does not depend on the type of electron beam production (scattering foil or swept scanned beam). Selection of isodose line for dose prescription is very critical for the value of the TCF and the dose coverage. The TCF is inversely proportional to the isodose value selected for the treatment and nearly linear with field size and beam energy. Air gap between applicator and the surface reduces the dose uniformity. Tertiary collimator moderately improves the lateral coverage for high energy beams.
To adequately cover the target volume in electron beam treatment, lateral and depth coverage should be considered. The coverage at depth is strongly dependent on the choice of isodose line or beam normalization. If the dose prescription is at dmax (i.e., the 100% isodose line is selected), the choice of beam energy is not critical for depth coverage since dmax is nearly independent of energy for smaller fields. The 100% isodose line should not be chosen for treatment because of the significant constriction of this isodose line and inadequate coverage at depth. For a higher TCF, a minimum air gap between the cone to the surface of the patient is desired. If such is not possible, then a tertiary collimator at the skin is required. Whenever, a tertiary collimator is used, it is advised to increase the collimator field size by a factor of 1.4.
提供一种用于电子束治疗中剂量处方的最佳射野大小和等剂量线选择的方法。
电子束剂量均匀性根据靶区覆盖因子（TCF）来定义，TCF是给定治疗体积剂量覆盖的一个指标。根据各种加速器的测量数据，研究了TCF与射野大小、束流能量以及处方等剂量水平的关系。研究了TCF对电子施源器/限光筒与表面之间气隙的影响。分析了来自散射箔和扫描束装置的电子束的靶区覆盖情况。
提供了一种数学方法，可根据TCF优化给定等剂量线对靶区的覆盖射野大小，TCF强烈依赖于加速器类型和准直器设计。对于给定类型的准直系统，TCF不依赖于电子束产生类型（散射箔或扫描束）。剂量处方等剂量线的选择对TCF值和剂量覆盖非常关键。TCF与为治疗选择的等剂量值成反比，与射野大小和束流能量近似呈线性关系。施源器与表面之间的气隙会降低剂量均匀性。三级准直器适度改善高能束的侧向覆盖。
为在电子束治疗中充分覆盖靶体积，应考虑侧向和深度覆盖。深度覆盖强烈依赖于等剂量线或束流归一化的选择。如果剂量处方在dmax处（即选择100%等剂量线），对于深度覆盖而言束流能量的选择并不关键，因为对于较小射野dmax几乎与能量无关。不应选择100%等剂量线进行治疗，因为该等剂量线明显收缩且深度覆盖不足。为获得更高的TCF，希望限光筒与患者表面之间的气隙最小。如果无法做到，则需要在皮肤处使用三级准直器。每当使用三级准直器时，建议将准直器射野大小增大1.4倍。
