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

Abstract

In IMRT, the method for a quantitative comparison of two-dimensional dose distributions is still under development. The gamma 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 y angle distribution and other tools (dose difference map, profiles, y 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 y vector calculations are based on a 3% dose criterion and a 3 mm DTA acceptance criterion. The mean value gamma(mean) (mean value in the y distribution) of the various IMRT plans is 0.45+/-0.10 (1 SD). On average, the percentage of points exceeding the acceptance criteria of gamma < or = 1 (gamma > 1) is 5.8+/-5.4% (1 SD). The mean value of gamma 1% (1% of points have an equal or higher gamma value) is 1.47+/-0.59 (1 SD) for IMRT plans. In 5 out of 27 planes, gamma > 1 is substantially larger than the average. This is also indicated in gamma area histograms. Planes with large areas outside the tolerance criteria were further evaluated using gamma angle distributions. This additional information indicates that the large areas with high gamma 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 gamma values (e.g. gamma area histogram), acceptance criteria for IMRT hybrid plans can be defined. For the interpretation of the gamma maps, distributions of the gamma angle and traditional evaluation methods, such as dose profiles, are still very useful.

Dosimetry tools and techniques for IMRT

“…The example shows the regions that fail the criteria ͑3% and 3 mm in this case͒, highlighting areas of significant failure. Stock et al 164 also identified the fact that the angle that the ␥ vector makes to the spatial axes ͑the complement to the angle the vector makes to the dose axis͒ can be used to determine whether the ␥ discrepancy is due to a spatial or dose error. An example of the ␥ angle is shown in Fig.…”

“…6͑b͒. 164 While the ␥ angle may be useful in evaluating the failure reasons, it is very sensitive to dose distribution noise and can be difficult to visually evaluate.…”

“…162,164,167,[175][176][177][178] One example is shown in Fig. 4͑f͒, provided from Childress and Rosen 162 based on their normalized agreement test ͑NAT͒.…”

NCS Report 22: Code of practice for the quality assurance and control for intensity modulated radiotherapy

“…We recommend performing a gamma evaluation with a criterion of at least 3%/3mm (96,102), if possible using absolute dosimetry. For a 2D and 3D evaluation all measured points with a dose below 10% of the reference dose should be discarded to avoid false positives due to low signal-to-noise in the low dose area.…”

Wiener Filter Used in the EBT2 Film for Radiation Therapy

“…For the effectiveness and success of the treatment prescribed,a high spatial and dosimetric accuracy during treatment delivery is of crucialimportance.Therefore, particular quality assurance (QA) procedureshave been developed together with those increasingly sophisticatedtreatment techniques [1]- [3], which wouldinclude the verification of absolute doses at one or more reference points as well as of twodimensionaldose distributions in different planes in awater-equivalent phantom.…”