The use of an electronic portal imaging system to measure portal dose and portal dose profiles

Parsaei, Homayoon; El‐Khatib, Ellen; Rajapakshe, Rasika

doi:10.1118/1.598379

Cited by 32 publications

(34 citation statements)

References 36 publications

(51 reference statements)

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“…4,11,12 The buildup is also likely to remove lower energy primary photons and reduce the offaxis response. This method does not account for the off-axis response of the EPID when subsequent dosimetry is performed with d max buildup.…”

Section: Pixel Sensitivity Variationmentioning

confidence: 98%

Correction of pixel sensitivity variation and off‐axis response for amorphous silicon EPID dosimetry

2005

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The aim of this work is to determine the pixel sensitivity variation and off-axis dose response of an amorphous silicon electronic portal imaging device (EPID), and develop a correction method to improve EPID dosimetry. The uncorrected or raw pixel response of the aS500 amorphous silicon EPID shows differences in response (sensitivity) of individual pixels as well as a large off-axis differential response with respect to an ion chamber in water. Both can be corrected by division of raw images by the flood-field (FF) image. However, this leads to two problems for dosimetry: (1) the beam profile is present in both the raw image and FF image, and hence is "washed out" of the corrected image, and (2) any mismatch of EPID position between dosimetry and FF calibration means that the beam profile and off-axis response in the raw image and FF are misaligned. This causes artifacts in FF division and dosimetric errors. A method was developed to measure the off-axis response and pixel sensitivity variation separately to allow correction of images at any EPID position while retaining beam profile information. The pixel sensitivity variation is applied to the imager plane and is independent of imager position. The off-axis response depends on the imager plane position relative to the beam central axis. The pixel sensitivities were derived from multiple images of the same symmetric field acquired with the detector displaced laterally between each image. The off-axis response was measured by acquiring off-axis raw images (FF correction removed) and dividing out the off-axis beam fluence and previously determined pixel sensitivity differences. The dosimetric errors due to lateral and vertical detector displacement with the conventional FF calibration method were measured and compared to the new method. Corrected EPID profiles were then compared to beam profiles measured with ion chamber in water for open fields. The EPID was found to have a large off-axis differential response with respect to an ion chamber in water, particularly for 6 MV. This increased to 13% at 15 cm off-axis for 6 MV, and 3.5% for 18 MV at the isocenter plane. The dosimetric errors introduced by detector displacement with conventional FF calibration were found to be approximately 1% per centimeter of lateral detector displacement and 0.1% per centimeter of vertical displacement. These were reduced to less than 1% for any position with the new correction method. Corrected EPID images agreed with ion-chamber measurements to within 2% (excluding penumbra and low-dose areas outside the field) for various field sizes. The new correction method gives consistent dosimetry for any EPID position and retains beam profile information in the image.

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Section: Pixel Sensitivity Variationmentioning

confidence: 98%

Correction of pixel sensitivity variation and off‐axis response for amorphous silicon EPID dosimetry

2005

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“…Outside the beam edge, an increase in the measured dose can be attributed to detector glare 44 as previously reported. Glare had also been observed with the SLIC-EPIDS 25 and is related to the difference in the absorption of low energy photons existing in the penumbra of the beam in the high Z material of the detector. The photon spectrum changes with distance from the central axis where the amount of soft radiation is proportionally greater.…”

Section: Pϭ426xdϫ1584 ͑2͒mentioning

confidence: 89%

“…[37][38][39][40][41][42] Previous work performed at our institution included the acquisition of portal dose transmission profiles measured with the SLIC-EPID. 25 We reported this technology to be suitable for transit dosimetry of static fields. However, dose verification in dynamic fields was not possible due to the method of image acquisition which consisted of scanning the ionization matrix row by row, by successively activating high voltage switches and measuring the currents in 256 column electrodes.…”

Section: Introductionmentioning

confidence: 98%

“…Its use for portal dosimetry was investigated and reported by de Boer et al 20,21 The use of the PortalVision ͑Varian Medical Systems, Palo Alto, CA͒. SLIC-EPID system for portal dose measurements has been investigated by Essers et al, 22 Yin et al, 23 Zhu et al, 24 and Parsaei et al 25 They showed the need to correct each pixel value for the variation in response of each detector element to a uniform flood field. The relationship between pixel value and portal dose then followed a square root response.…”

Section: Introductionmentioning

confidence: 98%

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An investigation of a new amorphous silicon electronic portal imaging device for transit dosimetry

2002

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The relationship between the pixel value and exit dose was investigated for a new commercially available amorphous silicon electronic portal imaging device. The pixel to dose mapping function was established to be linear for detector distances between 116.5 cm to 150 cm from the source, radiation field sizes from 5 x 5 cm2 to 20 x 20 cm2 and beam energies of 6 to 18 MV. Coefficients in the mapping function were found to be dependent on beam energy and field size. Open and wedged field profiles measured with the device showed agreement to a maximum of 5% and 8%, respectively, as compared to film. A comparison of relative transmission measurements between the EPID and ion chamber indicate a maximum deviation of 6% and 2% at 6 and 18 MV, respectively, for an attenuator thickness of 21 cm and SDD > or = 130 cm. It was found that accuracies of better than 1% could be obtained if detector position and field size specific fitting parameters were used to generate unique mapping functions for each configuration.

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“…These include megavoltage simulation and treatment of the obese patient 78 and efficient placement of lung blocks for total body irradiation 79 , 80 . Investigators have used the EPID for the design 81 – 83 and verification 84 , 85 of compensating filters.…”

Section: Epid Clinical Implementationmentioning

confidence: 99%

Guide to clinical use of electronic portal imaging

Herman

Kruse

Hagness

2000

J Applied Clin Med Phys

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The Electronic Portal Imaging Device (EPID) provides localization quality images and computer‐aided analysis, which should in principal, replace portal film imaging. Modern EPIDs deliver superior image quality and an array of analysis tools that improve clinical decision making. It has been demonstrated that the EPID can be a powerful tool in the reduction of treatment setup errors and the quality assurance and verification of complex treatments. However, in many radiation therapy clinics EPID technology is not in routine clinical use. This low utilization suggests that the capability and potential of the technology alone do not guarantee its full adoption. This paper addresses basic considerations required to facilitate clinical implementation of the EPID technology and gives specific examples of successful implementations. © 2000 American College of Medical Physics. PACS number(s): 87.53.–j, 87.57.–s

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The use of an electronic portal imaging system to measure portal dose and portal dose profiles

Cited by 32 publications

References 36 publications

Correction of pixel sensitivity variation and off‐axis response for amorphous silicon EPID dosimetry

Correction of pixel sensitivity variation and off‐axis response for amorphous silicon EPID dosimetry

An investigation of a new amorphous silicon electronic portal imaging device for transit dosimetry

Guide to clinical use of electronic portal imaging

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