Lam W C, Lam K S
Br J Radiol. 1983 Dec;56(672):949-56. doi: 10.1259/0007-1285-56-672-949.
To calculate dose distributions in off-axis planes for wedged fields, some commercial radiotherapy treatment planning systems use the Milan and Bentley method and simply modify the transverse principal-plane profile in the wedge direction with the value of its orthogonal open-field profile at the off-axis distance. For the Clinac 4 this simple modification may introduce serious distortions in off-axis dose distributions. The error can be considerably reduced if the basic beam data are supplemented with longitudinal principal-plane profiles and depth doses measured for the largest wedged field (14 X 32 cm2). These added data enable the determination of position-dependent effective attenuation coefficients of the wedges and hence, with the model proposed here, the calculation of transverse off-axis and longitudinal profiles as required. Even for very large field sizes and off-axis distances, the maximum deviation of generated profiles from measured ones varies from about 1% to the worst case of slightly above 4%. This procedure requires little extra computer time and data storage. The computed profiles are of sufficient accuracy for 3-dimensional treatment planning.
为了计算楔形野在离轴平面上的剂量分布，一些商业放射治疗治疗计划系统采用米兰和本特利方法，只需在离轴距离处用其正交开放野轮廓的值在楔形方向上修改横向主平面轮廓。对于 Clinac 4 型加速器，这种简单的修改可能会在离轴剂量分布中引入严重的畸变。如果用纵向主平面轮廓和为最大楔形野（14×32 cm²）测量的深度剂量补充基本束流数据，则误差可大大降低。这些补充数据能够确定楔形块的位置相关有效衰减系数，从而根据这里提出的模型，按要求计算横向离轴和纵向轮廓。即使对于非常大的野尺寸和离轴距离，生成的轮廓与测量轮廓的最大偏差从约 1%到略高于 4%的最坏情况不等。该过程几乎不需要额外的计算机时间和数据存储。计算出的轮廓对于三维治疗计划具有足够的精度。
