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 (C pm ) 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 C pm 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 C pm values for EPID indicate its higher efficiency than array.
Highlights
Treatment planning for static intensity modulated radiotherapy (s-IMRT), dynamic intensity modulated radiotherapy (d-IMRT) and volumetric intensity-modulated arc therapy (VMAT) were performed with Monaco version 3.20.01
The central axis point doses from EPID and ion chamber matched within 1.5% ± 0.3%
The pre-treatment verification of the s-IMRT, d-IMRT and VMAT plans have been carried out using ion chamber array and a-Si EPID routinely
Summary
Volumetric intensity-modulated arc therapy (VMAT), static intensity modulated radiotherapy (s-IMRT) and dynamic intensity modulated radiotherapy (d-IMRT) has been adopted as the preferred option for the treatment due to high dose conformity for the target and achieving high dose gradients around the periphery of the target, sparing the organs at risk (OAR).[1,2,3,4,5] These advanced techniques utilize the ability of multileaf collimator (MLC) to generate the arbitrary shapes and computer algorithm to develop treatment plan by optimization strategies such as simulated annealing.[6,7,8] This has led to an immense need for quality control in such complex planning and delivery of treatment plans. The planar measurements in phantom geometry provide an evaluation of the MLC and linear accelerator performance for a patient specific plan. It is a popular tool amongst the modern quality control techniques.[18,19] Besides positional verification[20,21] and quality control of linac[19,22,23, 24], several studies have validated a-Si EPID for patient specific dosimetry
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