Consistency analysis for the performance of planar detector systems used in advanced radiotherapy

Abstract

Purpose:To evaluate the performance linked to the consistency of a-Si EPID and ion-chamber array detectors for dose verification in advanced radiotherapy. Methods: Planar measurements were made for 250 patients using an array of ion chamber and a-Si EPID. For pre-treatment verification, the plans were generated on the phantom for re-calculation of doses. The γ-evaluation method with the criteria: dose-difference (DD) ≤ 3% and distance-to-agreement (DTA) ≤ 3 mm was used for the comparison of measurements. Also, the central axis (CAX) doses were measured using 0.125cc ion chamber and were compared with the central chamber of array and central pixel correlated dose value from EPID image. Two types of statistical approaches were applied for the analysis. Conventional statistics used analysis of variance (ANOVA) and unpaired t-test to evaluate the performance of the detectors. And statistical process control (SPC) was utilized to study the statistical variation for the measured data. Control charts (CC) based on an average ( ), standard deviation ( ̅ ) and exponentially weighted moving averages (EWMA) were prepared. The capability index (Cpm) was determined as an indicator for the performance consistency of the two systems. Results: Array and EPID measurements had the average gamma pass rates as 99.9% ± 0.15% and 98.9% ± 1.06% respectively. For the point doses, the 0.125cc chamber results were within 2.1% ± 0.5% of the central chamber of the array. Similarly, CAX doses from EPID and chamber matched within 1.5% ± 0.3%. The control charts showed that both the detectors were performing optimally and all the data points were within ± 5%. EWMA charts revealed that both the detectors had a slow drift along the mean of the processes but was found well within ± 3%. Further, higher Cpm values for EPID demonstrate its higher efficiency for radiotherapy techniques. Conclusion: The performances of both the detectors were seen to be of high quality irrespective of the radiotherapy technique. Higher Cpm values for EPID indicate its higher efficiency than array. FIG. 2(a-c): ̅ control charts for s-IMRT, d-IMRT and VMAT QA measurements by a-Si EPID and array FIG. 3: EWMA control charts for s-IMRT, d-IMRT and VMAT QA measurements by a-Si EPID and array