Figure 1.

Typical 2D gamma algorithm (a) interpolates the 3D dose grid to a 2D plane at the depth of measurement, and then performs the gamma comparison using 2D Euclidean distance on the interpolation plane and the dose difference. Accuracy of this 2D gamma comparison therefore depends on the veracity of the 2D interpolation. The 3D dose grid (blue ‘o’), the location of ion chamber measurement (red ‘x’), and the interpolated 2D dose grid (red ‘square’) are shown. Full 3D gamma algorithm (b) directly compares the measured dose to the calculated 3D dose grid using 3D Euclidean distance and the dose difference. Calculated 3D dose grid (blue ‘o’) and the point of measurement (red ‘x’) are shown. A 3D sphere of the same radius (i.e., 3 mm here) encompasses points on the 3D dose grid to be considered for gamma comparison with the measurement. This 3D gamma algorithm allows direct comparison of measured to calculated doses without resorting to interpolation.