Interpretation and evaluation of the γ index and the γ index angle for the verification of IMRT hybrid plans

In IMRT, the method for a quantitative comparison of two-dimensional dose distributions is still under development. The γ evaluation method proposed by Low et al is the most accepted approach and has been adapted by many groups. Based on the concept of Low et al we developed a software tool with an intelligent search algorithm to minimize the calculation time. For the interpretation of deviations a γ angle distribution and other tools (dose difference map, profiles, γ area histograms, etc) are integrated in the software package. Ten hybrid plans are included in the verification study containing 6 IMRT head and neck cases, 2 IMRT prostate cases and one IMRT paravertebral case as well as a standard uniform intensity conformal 4 field box treatment for comparison. IMRT plans are realized with a segmental MLC delivery technique. The fields of a hybrid plan are applied at once and dose distributions are measured with films in three planes of a verification phantom. All γ vector calculations are based on a 3% dose criterion and a 3 mm DTA acceptance criterion. The mean value γmean (mean value in the γ distribution) of the various IMRT plans is 0.45 ± 0.10 (1 SD). On average, the percentage of points exceeding the acceptance criteria of γ ≤ 1 (γ>1) is 5.8 ± 5.4% (1 SD). The mean value of γ1% (1% of points have an equal or higher γ value) is 1.47 ± 0.59 (1 SD) for IMRT plans. In 5 out of 27 planes, γ>1 is substantially larger than the average. This is also indicated in γ area histograms. Planes with large areas outside the tolerance criteria were further evaluated using γ angle distributions. This additional information indicates that the large areas with high γ values are dominated by the dose difference. It is shown that the deviations are influenced by tongue and groove effects. From the statistical evaluation of γ values (e.g. γ area histogram), acceptance criteria for IMRT hybrid plans can be defined. For the interpretation of the γ maps, distributions of the γ angle and traditional evaluation methods, such as dose profiles, are still very useful.