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ABSTRACT

Background: Although Electronic Portal imaging Devices (EPIDs) have originally developed for positioning verification, they can also be used for dosimetric purposes. In the current work, the dose response of minimum detectable thickness of a Scanning Liquid filled Ion Chamber EPID, SLIC-EPID, and the variation of transmitted dose with the shift of inhomogeneity inside of phantom was also evaluated were investigated.

Materials and Methods: The SLIC-EPID pixel values were converted to the dose values using ionization chamber calibration and KODAK Extended Dose Range films (EDR2 films). The variation of EPID dose values with phantom thickness was investigated. In order to find the rate of dose deposited per centimetre of phantom, several reference points were defined and the variation of dose delivered to the points in the vicinity of reference points was investigated. Two cm thick foam layer, as air gap, was shifted in the beam direction to evaluate the variation of transmitted dose with the shift of inhomogeneity position inside of phantom.

Results: An exponential decrease of the transmitted dose values was observed with the increase of the thickness of attenuators. The maximum and minimum rate of dose deposited per unit of phantom thickness was found to be 5.45% /cm and 3.78% /cm, respectively. Due to the reproducibility and noise level of SLIC-EPID, a 0.5 cm of thickness can be detected with a good reliability. The relative error of EPID dose values increases with an increase of phantom thickness for both data sets. The relative error did not exceed 0.7%. No significant variation in transmitted dose inplane and crossplane profiles were found with the shift of inhomogeneity in the beam direction.

 Conclusion: The minimum detectable thickness is an important factor to evaluate an imager for dosimetric purposes. The SLIC-EPID can be used as a reliable two-dimensional dosimeter.

 

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Background: In order to assign appropriate planning target volume (PTV) margins, each centre should measure the patient positioning deviations for their set-up techniques. At the Royal Marsden Hospital, UK, a conformal shell (cast) system is used when a stereotactic frame is not suitable. In this paper, we report on a series of measurements with the aim of obtaining the systematic and random components of positioning error when using the above-mentioned shell system. Materials and Methods: The verification protocol was based on orthogonal pairs of anterior-posterior and lateral electronic portal images (EPIs) used to check the isocentre position. The isocentre verification results of paediatric patients were analysed. A practical ‘off-line’ patient set-up correction strategy had been used with the aim of reducing systematic errors. The verification protocol involved EPI acquisition on the first three fractions and then on a weekly basis. Additional images were taken if an isocentre movement was applied based on a 3 mm tolerance for a consistent 1D discrepancy. Results: Four patients required isocentre corrections ranging between 2 mm and 4 mm. Following the off-line corrections, the residual systematic errors in each direction were within 0.5 mm while the 1D random variation was about 1.0 mm. Conclusions: The head fixation system in conjunction with the correction strategy successfully kept the random and systematic positioning errors within an acceptable level well within the 3 mm tolerance. The measured components of positioning error can be used to define appropriate PTV margins.

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Background: Electronic portal imaging devices (EPIDs) play an important role in radiation therapy portal imaging, geometric and dosimetric verifications. A successful utilization of EPIDs for imaging and dosimetric purposes requires a reliable quality control process routine to be carried out regularly. In this study, two in-house phantoms were developed and analyzed for implementation in a quality assurance program for dosimetry purposes. Materials and Methods: An amorphous silicon (a-Si) imager (OptiVue500) was used. A low contrast resolution phantom and an image quality phantom were constructed and implemented. Low contrast resolution of the EPID was evaluated by counting the number of holes detectable in the image of phantom using human observers and a software. The image quality phantom was used for modulation transfer function, contrast to noise ratio and noise level evaluations. This phantom contains five sets of high-contrast rectangular bar patterns of variable spatial frequencies and six uniform regions. Results: Although the manual low contrast resolution method was observer-dependent and insensitive to artifacts, the automatic method was robust and fully objective but sensitive to artifacts. The critical frequency values for 6 and 18 MV were 0.3558±0.006 lp/mm and 0.2707±0.006 lp/mm respectively. The contrast-to-noise ratio was found to be ~ 240% higher for 6 MV compared to 18 MV. Conclusion: The developed phantoms provide a convenient process for periodic performance of an EPID. These phantoms are independent of the EPID system and provide robust tools for continuous monitoring of image quality parameters as well as dosimetric parameters.