Room: AAPM ePoster Library
Purpose: purpose of this study was to develop an algorithm to detect the multi-leaf collimator (MLC) position error from a high sampling rate log file (HLF) recorded by an Elekta linac and to investigate the optimal collimator angle in a prostate volumetric modulated arc therapy (VMAT) plan.
Methods: prostate VMAT plans in which the collimator angle was changed every 5 degrees from 0 to 50 degrees were irradiated to be detected by the three-dimensional semiconductor detector (Delta4). At the same time, each HLF was acquired from the irradiation device and exported into our software. The areas related between the MLC positions and monitor units (MUs) were obtained from a radiotherapy planning system (RTPS) and HLF, respectively. The ratio between the two areas was defined as the evaluation index (EI). The relationship between the EI and dose evaluation indexes (DD; dose difference, DTA; distance to agreement, GPR; gamma pass ratio) of the Delta4 was investigated and the optimal collimator angle with a high irradiation accuracy was examined.
Results: parameter that most affected the collimator angle determined by our developed algorithm was DD. The collimator angle in which DD decreased the most was 35 degrees. The DDs at collimator angles 30, 35, and 40 degrees were less than 85%. The DTA and GPR exceeded 95% and 99% at all collimator angles, respectively. The correlation coefficient between EI and DD was 0.59.
Conclusion: cause of the decrease in DD with an increasing collimator angle was a mismatch between the setting of the minimum MLC gap on the RTPS and linac. The collimator angle in the prostate VMAT on the Elekta linac should be used less than 25 degrees to achieve a DD pass ratio greater than 85%.