Poirier Yannick, Kouznetsov Alexei, Koger Brandon, Tambasco Mauro
CancerCare Manitoba, 675 McDermot Ave, Winnipeg, Manitoba R3E 0V9, Canada.
Department of Physics and Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada.
Med Phys. 2014 Apr;41(4):041915. doi: 10.1118/1.4869159.
To introduce and validate a kilovoltage (kV) x-ray source model and characterization method to compute absorbed dose accrued from kV x-rays.
The authors propose a simplified virtual point source model and characterization method for a kV x-ray source. The source is modeled by: (1) characterizing the spatial spectral and fluence distributions of the photons at a plane at the isocenter, and (2) creating a virtual point source from which photons are generated to yield the derived spatial spectral and fluence distribution at isocenter of an imaging system. The spatial photon distribution is determined by in-air relative dose measurements along the transverse (x) and radial (y) directions. The spectrum is characterized using transverse axis half-value layer measurements and the nominal peak potential (kVp). This source modeling approach is used to characterize a Varian(®) on-board-imager (OBI(®)) for four default cone-beam CT beam qualities: beams using a half bowtie filter (HBT) with 110 and 125 kVp, and a full bowtie filter (FBT) with 100 and 125 kVp. The source model and characterization method was validated by comparing dose computed by the authors' inhouse software (kVDoseCalc) to relative dose measurements in a homogeneous and a heterogeneous block phantom comprised of tissue, bone, and lung-equivalent materials.
The characterized beam qualities and spatial photon distributions are comparable to reported values in the literature. Agreement between computed and measured percent depth-dose curves is ⩽ 2% in the homogeneous block phantom and ⩽ 2.5% in the heterogeneous block phantom. Transverse axis profiles taken at depths of 2 and 6 cm in the homogeneous block phantom show an agreement within 4%. All transverse axis dose profiles in water, in bone, and lung-equivalent materials for beams using a HBT, have an agreement within 5%. Measured profiles of FBT beams in bone and lung-equivalent materials were higher than their computed counterparts resulting in an agreement within 2.5%, 5%, and 8% within solid water, bone, and lung, respectively.
The proposed virtual point source model and characterization method can be used to compute absorbed dose in both the homogeneous and heterogeneous block phantoms within of 2%-8% of measured values, depending on the phantom and the beam quality. The authors' results also provide experimental validation for their kV dose computation software, kVDoseCalc.
介绍并验证一种千伏（kV）X射线源模型及表征方法，以计算千伏X射线累积的吸收剂量。
作者提出了一种针对千伏X射线源的简化虚拟点源模型及表征方法。该源通过以下方式建模：（1）表征等中心平面处光子的空间光谱和注量分布；（2）创建一个虚拟点源，从该点源产生光子，以产生成像系统等中心处的导出空间光谱和注量分布。空间光子分布通过沿横向（x）和径向（y）方向的空气相对剂量测量来确定。使用横向轴半值层测量和标称峰值电位（kVp）来表征光谱。这种源建模方法用于表征瓦里安（Varian®）机载成像仪（OBI®）的四种默认锥束CT光束质量：使用半蝴蝶结滤波器（HBT）且kVp为110和125的光束，以及使用全蝴蝶结滤波器（FBT）且kVp为100和125的光束。通过将作者内部软件（kVDoseCalc）计算的剂量与由组织、骨骼和肺等效材料组成的均匀和非均匀体模中的相对剂量测量值进行比较，验证了源模型和表征方法。
表征的光束质量和空间光子分布与文献报道的值相当。在均匀体模中，计算和测量的百分深度剂量曲线之间的一致性在2%以内，在非均匀体模中在2.5%以内。在均匀体模中2 cm和6 cm深度处获取的横向轴剖面显示一致性在4%以内。对于使用HBT的光束，在水、骨骼和肺等效材料中的所有横向轴剂量剖面的一致性在5%以内。在骨骼和肺等效材料中FBT光束的测量剖面高于其计算值，在固体水、骨骼和肺中的一致性分别在2.5%、5%和8%以内。
所提出的虚拟点源模型和表征方法可用于计算均匀和非均匀体模中的吸收剂量，其结果与测量值的偏差在2% - 8%以内，具体取决于体模和光束质量。作者的结果也为其千伏剂量计算软件kVDoseCalc提供了实验验证。
