A technique for the quantitative evaluation of dose distributions

Abstract

The commissioning of a three-dimensional treatment planning system requires comparisons of measured and calculated dose distributions. Techniques have been developed to facilitate quantitative comparisons, including superimposed isodoses, dose-difference, and distance-to-agreement (DTA) distributions. The criterion for acceptable calculation performance is generally defined as a tolerance of the dose and DTA in regions of low and high dose gradients, respectively. The dose difference and DTA distributions complement each other in their useful regions. A composite distribution has recently been developed that presents the dose difference in regions that fail both dose-difference and DTA comparison criteria. Although the composite distribution identifies locations where the calculation fails the preselected criteria, no numerical quality measure is provided for display or analysis. A technique is developed to unify dose distribution comparisons using the acceptance criteria. The measure of acceptability is the multidimensional distance between the measurement and calculation points in both the dose and the physical distance, scaled as a fraction of the acceptance criteria. In a space composed of dose and spatial coordinates, the acceptance criteria form an ellipsoid surface, the major axis scales of which are determined by individual acceptance criteria and the center of which is located at the measurement point in question. When the calculated dose distribution surface passes through the ellipsoid, the calculation passes the acceptance test for the measurement point. The minimum radial distance between the measurement point and the calculation points (expressed as a surface in the dose-distance space) is termed the gamma index. Regions where gamma > 1 correspond to locations where the calculation does not meet the acceptance criteria. The determination of gamma throughout the measured dose distribution provides a presentation that quantitatively indicates the calculation accuracy. Examples of a 6 MV beam penumbra are used to illustrate the gamma index.

Volumetric‐modulated arc therapy and intensity‐modulated radiation therapy treatment planning for prostate cancer with flattened beam and flattening filter free linear accelerators

“…A correction factor matrix was used to correct the matrices for angular dependencies, including couch attenuation. The sum of the corrected matrices was compared to the calculated dose by gamma evaluation36 with a dose tolerance of 3% of the maximum dose and a distance to agreement of 3 mm. The dose calculations in the TPS were performed with a dose grid of 1.5 mm in the measurement plane.…”

IMRT clinical implementation: Prostate and pelvic node irradiation using Helios and a 120‐leaf multileaf collimator

“…The advantage of this simple comparison is that large differences can be visualized immediately. A more accurate method may be to combine dose and distance differences into a single value called the gamma index of Low et al 7 …”

“…, 7 However, a general overview of some of the parameters required for the initiation of IMRT treatments will be described in order to explain our choice of both the Helios and MLC parameters. The rationale behind the use of IMRT for pelvic nodes and prostate treatment has already been described in detail, 4 and preliminary results from the RTOG trial 9413 suggest pelvic irradiation improves biochemical disease control compared with prostate radiotherapy alone 9 …”

Clinical usefulness of MLCs in robotic radiosurgery systems for prostate SBRT

“…As irradiated films were marked to indicate the position of the in‐house umbonate phantom, we aligned the position in the film analysis by matching the markings made for the umbos during measurements and calculations. Gamma‐index analysis 20 was used to compare the measurements and calculations, by applying a criterion of 3% local pixel dose difference (LPDD)/2 mm distance‐to‐agreement (DTA) and a threshold of 30% of the maximum dose. This criterion was determined based on previous studies,21, 22, 23 and the threshold was set according to the dose constraints of the critical organs.…”