Statistical analysis of dynamic arc therapy in prostate localizations

Introduction In order to optimize the quality control (QC) of the treatment chain, a statistical analysis of the results for isocentric double arc dynamic therapy localized to the prostate has been performed. Material and method From January 2011 to October 2012, 291 such treatments were planned on the Trilogy TM (Varian). Ionisation chamber measurement of the dose delivered to the isocentre in a phantom for the patient's beams has been compared to the dose calculated in this phantom. The comparison of the dose distribution measured and calculated in the DELTA4 TM (Scandidos) for these same beams has been evaluated in terms of median dose deviation and global gamma index (3%/3 mm). Results and analysis Each arc being measured individually, the mean of the dose deviations at the isocentre for the 183 plans treated in 2011 was –0.33% (s = 2.5%). In the cumulative approach (whole plan) it was –0.35% (s = 1.98%). In 2012 (108 plans), arc by arc, the mean of the dose deviations at the isocentre was 0.57% (s = 1.9%) and the mean dose deviations on the detectors matrix showed a mean of –0.22% (s = 0.6 %). The mean percentage of points were the gamma index did not exceed 1 was 99.92% (s = 0.2%). In the global approach, these values were –0.15% (s = 1.18%) et de 99.86% (s = 0.47%) respectively. These results were consistent in terms of point dose as well as 2D dose distribution whatever the approach and showed a good agreement between measured and expected dose. Conclusion Systematic patient related QC for arc dynamic plans involving the prostate area only have been stopped. Monthly stability and reproducibility of the treatment chain is performed based on measurement of the same test patient and an on going treatment selected randomly.