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Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator
Published on: May 9, 2014
Characterization of a novel large-area electronic portal imaging device system for a ring-shaped HalosTx linear
Guyu Dai1, Dong Gao2, Qing Xiao1
1Department of Radiotherapy Physics & Technology, West China Hospital, Sichuan University, Chengdu, China.
Background:
Electronic portal imaging device (EPID)-based in vivo dosimetry (IVD) has attracted considerable interest in recent years. Nevertheless, the limited detection area of conventional EPID systems has hindered their broad clinical adoption. Therefore, this study evaluated a novel large-area EPID (65 cm × 61 cm) integrated into the HalosTx linear accelerator (linac), focusing on stability, imaging quality, field coverage, and error sensitivity for radiotherapy quality assurance (QA).
Methods:
Basic physical characteristics, including short-term repeatability, dose-response linearity, the influence of gantry angle on dose, and dose-rate dependencies, were first tested. Megavolt (MV) imaging quality tests were then performed to evaluate scaling, spatial resolution, uniformity, contrast, and noise. Subsequently, field coverage and sensitivity were tested across various treatment plans. Additionally, patient setup errors for a breast cancer treatment plan were simulated using a female thorax phantom to explore the sensitivity of the system to setup errors.
Results:
The EPID demonstrated high stability in basic physical characteristics, with maximum deviations of 0.25% for short-term repeatability, 0.6% for dose-response linearity, 0.757% for gantry angle influence, and 0.372% for dose-rate dependency. The results of scaling, spatial resolution, uniformity, contrast, and noise were 0 mm, 0.31 lp/mm, 99.39%, 0.74, and 161.33, respectively. Benefitting from the 65 cm × 61 cm detector effective area, this EPID successfully monitored all planning beam fields across various treatment plans. Conversely, the Vital Beam and Versa HD systems could only monitor 60% of the beam fields for breast cancer. The sensitivity to machine-related errors was calculated to be 0.2 mm for jaw position, 0.2 mm for multileaf collimator (MLC) position, 0.1% for linac output, 0.5° for collimator rotation. The phantom test for breast cancer highlighted the unique angle-dependent gamma analysis of this EPID system, wherein each beam was subdivided for further analysis. This approach revealed notably high sensitivity to setup errors, particularly in the angular ranges of 56.1°-140° clockwise and 140°-78° counterclockwise.
Conclusions:
Our preliminary findings indicate that the new EPID system exhibits good stability, favorable imaging quality, broad field coverage, and high sensitivity, suggesting its potential to facilitate novel EPID-based applications in radiotherapy.
