Analysis on dosimetric verification of flattening filter free conventional segmentation partial arc RapidArc plans

Abstract

By comparing the application of aSi-1200 EPID and Octavius 729 detector using dose verification of conventional segmentation flattening filter free partial RapidArc plans, the limitations and γ response to the modified multi-leaf collimator (MLC) model were investigated. CU (Calibration unit) value curves of Crossline and Inline in fluence plots for different square field plans designed on water phantom were used to examine the effectiveness of the backscattering shielding layer of an aSi-1200 EPID. A two-dimensional (2D) γ analysis was performed on 50 plans under different tolerance criteria and thresholds (global normalization). The plan data with lower γ pass rate were imported into Verisoft software for dose reconstruction. By introducing transmission factor (TF) and dosimetric leaf gap (DLG) deviations with different magnitudes in multi-leaf collimator (MLC) model, a total of 80 modified dose distributions were obtained, and the γ passing rates between measured doses of the original plans and the TPS doses of the modified plans were analyzed. The results show that the CU values in the areas with dropped fields are 2~3 times higher in the positive off-axis positions, and the measured values in the Inline directions were generally smaller than the Crossline directions for a Varian EDGE linac. Nevertheless, CU values of the Halcyon accelerator coincided almost completely in both directions. The median and mean γ passing rates obtained from the EPID were higher than the 729 matrix. The deviations DD% between the TPS doses and the reconstructed doses based on measurements from the Octavius 729 and Octavius 1500 matrices are as follows: D98% of PGTV (-9.60%，-14.30%)，V30 of heart (-6.80%，-2.36%), V30 of Lung L (-8.11%，15.79%), V30 of Lung R (100%，50%), Dmax of Cord (3.55%, -‍0.82%). With the increase in the deviations of TF and DLG, the γ pass rates obtained from the EPID and Octavius 729 showed no trend of decreasing. A method to test the effectiveness of the backscattering shielding of an EPID detector performed using linac is provided. The dose measurement results suggested that the γ index obtained from the aSi-1200 EPID and Octavius 729 detectors could not reliably identify an error-introducing MLC model, which provide reference for the interpretation of the reported high failure rates of the IROC-H phantom tests.

Keywords

• conventional segmentation
• partial arc
• flattening filter free
• dosimetric verification