Evaluation of Gamma Index Analysis for Detecting Errors in Patient-specific Quality Assurance in Intensity Modulated Radiotherapy.

Quality assurance practices performed before treatment are believed to identify various potential errors. In this study, 2-dimensional (2D) dosimetric results were analyzed by making some intentional mistakes in six different treatment plans. In this way, the detectability of errors was investigated. In all segments of all treatment plans, one of the multileaf collimators was kept fixed at different positions on the central axis. In addition to multileaf collimators error, gantry error was also examined in the study. The dose distribution results obtained by Treatment Planning System (TPS) were compared with those obtained by the 2D array device, both as local calculation and global calculation methods, using the gamma analysis method. When the results are examined in the case where the Multi-leaf collimators (MLC) is fixed at the 1 cm position, the gamma analysis pass rate of the other plans, except two plans in the MD criterion, are calculated above the 95% limit. When the dose distributions obtained as a result of irradiation with 0.5-degree erroneous gantry angle were analyzed, it was found that all plans were at an acceptable rate in the maximum dose criterion. In case of incorrect irradiation, dose reduction or overdose may occur in the irradiated target area, even if gamma analysis pass rates are sufficient. Quality control procedures performed before treatment may be less effective. It is suggested that it would be better to examine the gamma analysis criteria applied for error detection with different values and local calculation method, and that device-based quality controls should be performed frequently.