The gamma evaluation method as a routine QA procedure of IMRT

Abstract

Background

The conventional QA procedures dedicated to 3D CRT are unsatisfactory if the dMLC is in operation. In the case of IMRT not only should the dose on the beam axis, but also its distribution in the total plane perpendicular to the beam be taken under control. The comparison between the predicted and the observed fluence can be achieved using the gamma method. It takes into consideration the dose difference and the spatial displacement between analyzed points to provide a γ-index as a result of comparison.

Aim

The aim of the investigation was to develop the procedure of IMRT verification based on the gamma algorithm.

Materials and Methods

700 patients have been irradiated using IMRT since 2002. Over 1500 images recorded on the film and/or EPID have been analyzed with the help of self-made software. Histograms of γ-value and the γ-images have been created for each field. The fields have been classified depending on tumour location and the method of dose delivery, to obtain an average result for each class. We have performed a comparison of γ-histograms acquired with the help of different methods of recording.

Results

We have observed a correlation between results of verification obtained with the help of the gamma algorithm and the method of intensity modulation.

Conclusion

Gamma evaluation allows one to find local hot-spots caused by irregularities in leaf motion or the tongue-and-groove effect.