Dose‐response characteristics of an amorphous silicon EPID

IntroductionA robust image quality assurance and analysis methodology for image‐guided localization systems is crucial to ensure the accurate localization and visualization of target tumors. In this study, the long‐term stability of selected image parameters was assessed and evaluated for the cone‐beam computed tomography (CBCT) mode, planar radiographic kV mode, and the radiographic MV mode of an Elekta VersaHD.Materials and MethodsThe CATPHAN, QckV‐1, and QC‐3 phantoms were used to evaluate the image quality parameters. The planar radiographic images were analyzed in PIPSpro™ with spatial resolution (f30, f40, f50), contrast to noise ratio (CNR) and noise being recorded. For XVI CBCT, Head and Neck Small20 (S20) and Pelvis Medium20 (M20) standard acquisition modes were evaluated for uniformity, noise, spatial resolution, and HU constancy. Dose and kVp for the XVI were recorded using the Unfors RaySafe Xi system with the R/F low detector for the kV planar radiographic mode. For each metric, values were normalized to the mean and the standard deviations were recorded.ResultsA total of 30 measurements were performed on a single Elekta VersaHD linear accelerator over an 18‐month period without significant adjustment or recalibration to the XVI or iViewGT systems during the evaluated time frame. For the planar radiographic spatial resolution, the normalized standard deviation values of the f30, f40, and f50 were 0.004, 0.003, and 0.003 and 0.015, 0.009, and 0.017 for kV and MV, respectively. The average recorded dose for kV was 67.96 μGy. The standard deviations of the evaluated metrics for the S20 acquisition were 0.083(f30), 0.058(f40), 0.056(f50), 0.021(Water/poly‐HU constancy), 0.029(uniformity) and 0.028(noise). The standard deviations for the M20 acquisition were 0.093(f30), 0.043(f40), 0.037(f50), 0.016(Water/poly‐HU constancy), 0.010(uniformity) and 0.011(Noise).ConclusionA study was performed to assess the stability of the basic image quality parameters recommended by TG‐142 for the Elekta XVI and iViewGT imaging systems. The two systems show consistent imaging and dosimetric properties over the evaluated time frame.

Section: Resultssupporting

confidence: 78%

“…1. This arm allows the detector to be positioned at source to electronic portal imaging device (EPID) distance of 160 cm with an active imaging area of 41 × 41 cm 9. The image matrix is created from an array of 1024 × 1024 photodiodes with a pitch of 400 μm 10.…”

Section: Materials/methodsmentioning

confidence: 99%

An evaluation of the stability of image quality parameters of Elekta X‐ray volume imager and iViewGT imaging systems

Stanley

Rasmussen

Kirby

et al. 2018

“…These factors resulted independent of the beam energies and of the MU/min used within 0.5% (2 SD), in agreement with the findings obtained in other works, for this energy range. ( 5 ) It must be noted that with respect to the normalization value at 100 MU, the mean EPID response for 1000 MUs was only 1% higher. Therefore, in this dose range, no saturation effect was observed.…”

Section: Resultsmentioning

confidence: 95%

“…A more detailed description of the functionality and basic properties of such devices is reported in the literature. ( 4 , 5 ) Above the detector, a copper plate with a thickness of 1 mm, acts as build‐up material, and the copper plate source–EPID distance (SED) is fixed around 159 cm (Table 1). However, the EPID can be in a retracted position when its use is not required.…”

Section: Methodsmentioning

confidence: 99%

Correlation functions for Elekta aSi EPIDs used as transit dosimeter for open fields

Cilla

Fidanzio

Greco

et al. 2010

In‐vivo dosimetry techniques are currently being applied only by a few Centers because they require time‐consuming implementation measurements, and workload for detector positioning and data analysis. The transit in‐vivo dosimetry performed by the electronic portal imaging device (EPID) avoids the problem of solid‐state detector positioning on the patient. Moreover, the dosimetric characterization of the recent Elekta aSi EPIDs in terms of signal stability and linearity make these detectors useful for the transit in‐vivo dosimetry with 6, 10 and 15 MV photon beams. However, the implementation of the EPID transit dosimetry requires several measurements. Recently, the present authors have developed an in‐vivo dosimetry method for 3D CRT based on correlation functions defined by the ratios between the transit signal, normalsnormalt(w,L), by the EPID and the phantom midplane dose, normalDnormalm(w,L), at the source to axis distance (SAD) as a function of the phantom thickness, w, and the square field dimensions, L. When the phantom midplane was positioned at distance, d, from the SAD, the ratios normalsnormalt(w,L)/snormalt'(d,w,L) were used to take into account the variation of the scattered photon contributions on the EPID as a function of d and L.The aim of this paper is the implementation of a procedure that uses generalized correlation functions obtained by nine Elekta Precise linac beams. The procedure can be used by other Elekta Precise linacs equipped with the same aSi EPIDs, assuming the stabilities of the beam output factors and the EPID signals. The procedure here reported avoids measurements in solid water equivalent phantoms needed to implement the in‐vivo dosimetry method in the radiotherapy department. A tolerance level ranging between ±5% and ±6% (depending on the type of tumor) was estimated for the comparison between the reconstructed isocenter dose, Diso, and the computed dose, Diso,TPS, by the treatment planning system (TPS).PACS number: 87.55.Qr; 87.56.Fc

“…There are issues related to the non‐water equivalence of the EPID 2 , 3 , 4 , 5 and the detector's image acquisition process 6 , 7 , 8 . Previous studies of the dose response characteristics of a‐Si EPIDs have reported an underresponse at small monitor unit (MU) exposures relative to longer exposures 9 , 10 , 11 , 12 , 13 . The resulting nonlinear EPID dose response, referred to here as gain ghosting, has been attributed to trapped charge effects 9 , 13 , 14 , 15 .…”

Section: Introductionmentioning

confidence: 99%

Dose calibration of EPIDs for segmented IMRT dosimetry

Deshpande

Xing

Holloway

et al. 2014

The purpose of this study was to investigate the dose response of amorphous silicon (a‐Si) electronic portal imaging devices (EPIDs) under different acquisition settings for both open jaw defined fields and segmented intensity‐modulated radiation therapy (IMRT) fields. Four different EPIDs were used. Two Siemens and one Elekta plus a standalone Perkin Elmer research EPID. Each was operated with different acquisition systems and settings. Dose response linearity was measured for open static jaw defined fields and ‘simple’ segmented IMRT fields for a range of equipment and system settings. Six ‘simple’ segmented IMRT fields were used. The segments of each IMRT field were fixed at 10×10.15emcm2 field size with equal MU per segment, each field having a total of 20 MU. Simultaneous measurements with an ionization chamber array (ICA) and EPID were performed to separate beam and detector response characteristics. Three different pixel calibration methods were demonstrated and compared for an example ‘clinical IMRT field’. The dose response with the Elekta EPID for ‘simple’ segmented IMRT fields versus static fields agreed to within 2.5% for monitor unit (MU)≥2. The dose response for the Siemens systems was difficult to interpret due to the poor reproducibility for segmented delivery, at MU≤5, which was not observed with the standalone research EPID nor ICA on the same machine. The dose response measured under different acquisition settings and different linac/EPID combinations matched closely (≤1%), except for the Siemens EPID. Clinical IMRT EPID dosimetry implemented with the different pixel‐to‐dose calibration methods indicated that calibration at 20 MU provides equivalent results to implementing a ghosting correction model. The nonlinear dose response was consistent across both clinical EPIDs and the standalone research EPID, with the exception of the poor reproducibility seen with Siemens EPID images of IMRT fields. The nonlinear dose response was relatively insensitive to acquisition settings and appears to be primarily due to gain ghosting effects. No additional ghosting correction factor is necessary when the pixel‐to‐dose calibration factor at small MU calibration method is used.PACS numbers: 87.53.Bn, 87.55.Qr, 87.56.Fc, 87.57.uq