Abstract
Purpose: To study the dosimetric characteristics of amorphous silicon Electronic Portal Imaging Device EPID and 2D array detector for dose verification of radiotherapy treatment plans, and the quality assurance QA testing of IMRT was investigated. Materials and methods: All measurements were done with Varian IX linear accelerator, aSi-1000 EPID and 2D array detector. The dose linearity, reproducibility, output factors, dose rate, SDD and response with slap phantom thickness have been measured and compared against those measured by ion chamber. Results: The characteristics of EPID and 2D array: the response of EPID agreed with 2D array and ion chamber 0.6cc. EPID and 2D array showed short-term output reproducibility with SD = 0.1%. The dose rates of 2D array SD = ±0.7%, EPID = ±0.4% compared with a 0.6 cc SD = ±0.5%. Output factor measurements for the central chamber of the EPID and 2D array showed no considerable deviation from ion chamber measurements. Measurement of beam profiles with the EPID and 2D array matched very well with the ion chamber measurements in the water phantom. The EPID is more sensitive to lower energy photons by increasing solid water phantom thickness. The mean and standard deviation passing rates (γ%≤1) for film, 2D array and EPID for 30 IMRT fields of five patients were 95.93 ± 0.96%, 99.05 ± 0.24%, and 99.37 ± 0.12%, respectively. Conclusion: The study shows that EPID and 2D array are a reliable and accurate dosimeter and a useful tool for quality assurance. We found that the EPID was more accurate compared with both 2D array and ion chamber. The gamma criterion of 3%/3 mm is the most suitable criteria for IMRT plans of QA.
Highlights
The quality assurance (QA) procedure in radiotherapy generally demands dose measurement as well as patient positioning check
The disadvantages of the 2D array system are: the low resolution of the detectors, the time taken to set up the detectors, phantom and connect to the external computer system with analysis software
The results showed that both of 2D array and EPID can be used in patient specific QA measurements for intensity modulated radiotherapy (IMRT)
Summary
The quality assurance (QA) procedure in radiotherapy generally demands dose measurement as well as patient positioning check. Ionization chamber array has become the standard device for quality assurance measurements in modern radiotherapy. The possibility of producing complex fields and dose shaping using devices such as multi leaf collimators (MLCs) has improved conformal radiotherapy techniques and boosted the clinical implementation of intensity modulated radiotherapy (IMRT) [2]. The increased complexity of clinical treatments raises the need for more accurate dose verification systems and procedures. Markovic M.et al (2014) [4] evaluated the Octavius Detector 1000 is an accurate, precise, and reliable detector, very useful for the daily performance of the patient specific quality assurance of radiotherapy treatment plans. EPID and 2D array have become the standard devices for QA measurements in modern radiotherapy techniques such as IMRT [5] [6]. The precision of dose delivery using linear accelerators to a patient must be accurate to within ±2% [7] [8]
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