Cadman P
Department of Medical Physics, Tom Baker Cancer Centre, Calgary, Alberta, Canada.
Med Phys. 1995 Apr;22(4):457-63. doi: 10.1118/1.597476.
An investigation of the scattering effects of dual asymmetric collimators on open fields is presented. It has been determined that the output measured in air and in a phantom is radially symmetric and that the scatter as a function of field size is independent of field center offset, for a wide variety of square and rectangular field shapes. The analysis has led to a new and simplified method to represent primary off-center ratios (POCRs), which requires a minimum set of symmetric field data, including beam profiles, collimator and phantom scatter factors, and tissue maximum ratios. The field edge correction (FEC) method proposed has been shown to accurately model the POCRs to a distance of 17.5 cm from the collimator rotation axis in a plane perpendicular to the isocenter and at depths up to 30 cm without the need for extrapolation of the POCRs or special measurements setups, which may be required for POCR modeling techniques that measure narrow beam attenuation coefficients.
本文介绍了双不对称准直器对开放野散射效应的研究。已经确定，对于各种方形和矩形野形状，在空气中和模体中测量的输出是径向对称的，并且作为野大小函数的散射与野中心偏移无关。该分析导致了一种新的简化方法来表示原射线离轴比（POCR），该方法需要最少的一组对称野数据，包括射束轮廓、准直器和模体散射因子以及组织最大剂量比。所提出的野边缘校正（FEC）方法已被证明能够在垂直于等中心的平面内，在距准直器旋转轴17.5 cm的距离处以及深度达30 cm时，准确地对POCR进行建模，而无需对POCR进行外推或采用特殊的测量设置，而这些对于测量窄束衰减系数的POCR建模技术可能是必需的。
