Comparison of two-dimensional transmitted dose maps: evaluation of existing algorithms.

Composite analysis and the gamma function are often used to assess the agreement between a reference and an evaluated two-dimensional dose maps. The intent of the study is to compare advantages, disadvantages and limitations of dose evaluation tools reported in the literature. In addition, in order to improve the gamma function output, a "Signed Matrix" was introduced using the ratio of relative dose difference maps. Transmitted dose maps were acquired for a range of homogeneous phantoms using Extended Dose Range (EDR2) films and a Scanning Liquid Ionization Chamber Electronic Portal Imaging Device (SLIC-EPID). For inhomogeneous case, the transmitted dose maps were obtained from EDR2 films measurement and a Treatment Planning System (TPS). The corresponding dose maps were compared based on composite and gamma function algorithms. The results showed that the agreement between reference and evaluated dose maps for the composite analysis were generally greater than those obtained using the gamma function. For homogeneous phantom comparison, the difference between the agreeing fractions calculated using composite analysis and gamma function increases with the increase of phantom thickness for deltaD = 0.5% and 1%. For inhomogeneous cases, a significant difference (approximately 5% for deltaD = 1.5%) was observed between the percentage agreement as calculated by composite and gamma function techniques. The concept of the composite model is closer than gamma function to the idea of the two-dimensional dose verification protocol proposed originally by van Dyk. However, the composite model results only display the passed or failed regions in the dose maps. On the other hand, the gamma function provides continuos information by distinguishing the points within each region. The overdosed/underdosed regions (the ratio of reference and evaluated doses at a given point) and the direction of the misalignment can be recognized with the enhanced gamma map convolved with a "Signed Matrix".