Normalize the response of <scp>EPID</scp> in pursuit of linear accelerator dosimetry standardization

Cai, Bin; Goddu, S; Yaddanapudi, S; Wen, Jie; Noël, C.; Mutic, Sasa; Sun, Baozhou

doi:10.1002/acm2.12222

Cited by 13 publications

(42 citation statements)

References 25 publications

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“…Similar to Cai et al. 's 16 work, we found that the PSM was stable at the

<

1% level for measurements separated by 3 months. Cai reported energy/modality dependence for the PSM, but we found the PSM to be energy/modality independent for photons.…”

Section: Discussionsupporting

confidence: 91%

“…For fixed radiation fields, we also intercompare field measurements spanning a range of lateral offsets that could be attained by the imager to confirm position independence. This, in effect, cross‐validates our method with those of Greer et al 13 and Cai et al, 16 who inter‐related shifted EPID measurements to derive the PSM. For this series of measurements, the imager was placed at the isocenter plane and the jaws adjusted to produce long‐narrow beams (slits) on the EPID imager.…”

Section: Methodssupporting

confidence: 82%

“…We found that single pixel misalignment in the imager shift would result in

\geq

1% PSM deviations in 6% of the EPID panel pixels; a 0.1% machine output variation between measurements would result in up‐to 5% pixel sensitivity errors; and that a 0.1% background offset (mimicking ghosting) could result in

\geq

5% errors in 85% of EPID pixels. Cai et al 16 . utilized sequential calibration field and dark‐field irradiations to minimize the effect of the ˜1% image ghosting and small (1 cm) imager offsets to minimize the change in scatter conditions inherent to the aS1000 imager (which has non‐uniform backscatter).…”

Section: Discussionmentioning

confidence: 99%

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A regression‐based approach to compute the pixels sensitivity map of linear accelerator portal imaging devices

2021

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Purpose To determine the pixel sensitivity map (PSM) for amorphous silicon electronic portal imaging devices (EPIDs) using a single flood field signal. Method and materials A raw EPID signal results from the incident particle energy fluence, the inherent pixels response, and the background signal. In large open fields, particle energy fluence is a slow‐varying signal that is locally considered spatially constant. Pixels response is a fast and abrupt varying behavior. The background signal is due to the EPID panel electronics, which is determined during radiation absence. To determine the PSM, after correcting for the background signal, we apply a model that captures the underlying smooth particle energy fluence‐induced signal. This fluence signal‐fitted model is then used to determine the PSM. Here, we use a polynomial‐based regression surface model in both x and y dimensions. To validate the generated PSM, we measure beams and compute PSMs for multiple beam energies with and without flattening filters and for multiple source‐to‐imager distances. Since the PSM is a detector characteristic, it should be independent of those variables. We also intercompare measurements of fixed slit fields with the EPID being shifted between measurements. Results The fluence signal of the flattening filter‐free (FFF) beams was optimally modeled as a 12th degree polynomial surfaces, which had ≤0.1% residuals near the central axis. The 6 and 10 MV FFF PSMs were within ˜0.1%, and independent of the EPID SID, suggesting that the PSM is energy independent. The 6, 10, and 15 MV flattened‐beam PSMs were well modeled as 12th degree polynomial surfaces, which were equivalent within ˜0.24% but differed from the FFF PSM by up to 0.5% near the beam central axis. Applying the FFF PSMs to the flattened‐beam measurements reduced the central‐axis deviation between the raw and corrected signal to <0.1%, confirming the PSM energy independence hypothesis. When the FFF PSM is utilized, output verification with shifted slit deliveries agreed within ˜0.5% for all beam energies, which is within the radiation delivery uncertainty of ˜0.57%. Conclusion PSM for MV EPIDs can be determined by separating out the slowly varying, well‐behaved fluence signal from the pixel‐to‐pixel sensitivity variations. The quality of the PSM is found to be dependent on the quality of the surface fit, which is best for the 6 MV FFF beam measured at SID equal to 180 cm. Within fitting errors, the PSM is independent of beam energy for 6, 10, and 15 MV beams with and without flattening filters. The PSM generation does not require shifting the EPID panel nor multiple EPID panel irradiations and should be usable for linacs with fixed geometry EPIDs.

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“…Similar to Cai et al. 's 16 work, we found that the PSM was stable at the

<

1% level for measurements separated by 3 months. Cai reported energy/modality dependence for the PSM, but we found the PSM to be energy/modality independent for photons.…”

Section: Discussionsupporting

confidence: 91%

Section: Methodssupporting

confidence: 82%

“…We found that single pixel misalignment in the imager shift would result in

\geq

\geq

Section: Discussionmentioning

confidence: 99%

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A regression‐based approach to compute the pixels sensitivity map of linear accelerator portal imaging devices

2021

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“…Specifically, the work of Yaddanapudi et al 9. provides a method of checking photon beam symmetry using PSM‐corrected EPID images for linac acceptance testing purposes and the work of Bin Cai et al 10. highlights the advantages of PSM‐corrected EPID imaging for beam profile analysis.…”

Section: Introductionmentioning

confidence: 99%

A proposed method for linear accelerator photon beam steering usingEPID

Barnes

Menk

Lamichhane

et al. 2018

J Applied Clin Med Phys

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Beam steering is the process of calibrating the angle and translational position with which a linear accelerator's (linac's) electron beam strikes the x‐ray target with respect to the collimator rotation axis. The shape of the dose profile is highly dependent on accurate beam steering and is essential for ensuring correct delivery of the radiotherapy treatment plan. Traditional methods of beam steering utilize a scanning water tank phantom that makes the process user‐dependent. This study is the first to provide a methodology for both beam angle steering and beam translational position steering based on EPID imaging of the beam and does not require a phantom. Both the EPID‐based beam angle steering and beam translational steering methods described have been validated against IC Profiler measurement. Wide field symmetry agreement was found between the EPID and IC Profiler to within 0.06 ± 0.14% (1 SD) and 0.32 ± 0.11% (1 SD) for flattened and flattening‐filter‐free (FFF) beams, respectively. For a 1.1% change in symmetry measured by IC Profiler the EPID method agreed to within 0.23%. For beam translational position steering, the EPID method agreed with IC Profiler method to within 0.03 ± 0.05 mm (1 SD) at isocenter. The EPID‐based methods presented are quick to perform, simple, accurate and could easily be integrated with the linac, potentially via the MPC application. The methods have the potential to remove user variability and to standardize the process of beam steering throughout the radiotherapy community.

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“…One factor is that EPID is mounted on the linac by means of a support arm, and the low-energy scatter ray produced by the interaction between X-rays and the support arm may lead to asymmetry of the EPID image in the inline direction [13,[21][22][23][24][25]; relevant studies show that the effect may exceed 5% [26,27]. The other factor is that EPID requires dark field and flood field calibration to ensure that the response of each pixel is constant, which results in the off-axis response of EPID being different from the actual output of the linac [28,29]. Therefore, when EPID is used as a dosimetry tool, these two factors must be corrected before converting pixel values into dose values.…”

Section: Introductionmentioning

confidence: 99%

Development of an Electronic Portal Imaging Device Dosimetry Method

et al. 2021

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Support arm backscatter and off-axis effects of an electronic portal imaging device (EPID) are challenging for radiotherapy quality assurance. Aiming at the issue, we proposed a simple yet effective method with correction matrices to rectify backscatter and off-axis responses for EPID images. First, we measured the square fields with ionization chamber array (ICA) and EPID simultaneously. Second, we calculated the dose-to-pixel value ratio and used it as the correction matrix of the corresponding field. Third, the correction value of the large field was replaced with that of the same point in the small field to generate a correction matrix suitable for different EPID images. Finally, we rectified the EPID image with the correction matrix, and then the processed EPID images were converted into the absolute dose. The calculated dose was compared with the measured dose via ICA. The gamma pass rates of 3%/3 mm and 2%/2 mm (5% threshold) were 99.6% ± 0.94% and 95.48% ± 1.03%, and the average gamma values were 0.28 ± 0.04 and 0.42 ± 0.05, respectively. Experimental results verified our method accurately corrected EPID images and converted pixel values into absolute dose values such that EPID was an efficient radiotherapy dosimetry tool.

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Normalize the response of EPID in pursuit of linear accelerator dosimetry standardization

Cited by 13 publications

References 25 publications

A regression‐based approach to compute the pixels sensitivity map of linear accelerator portal imaging devices

A regression‐based approach to compute the pixels sensitivity map of linear accelerator portal imaging devices

A proposed method for linear accelerator photon beam steering usingEPID

Development of an Electronic Portal Imaging Device Dosimetry Method

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