The use of an electronic portal imaging device for exit dosimetry and quality control measurements

Kirby, Mike C; Williams, Peter C.

doi:10.1016/0360-3016(94)00388-2

Cited by 91 publications

(38 citation statements)

References 32 publications

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“…It should be noted that this is an essential difference to methods which determine the exit dose from a portal image. [8][9][10][11] We have made a study of relevant scintillator properties for this purpose. Differences and similarities with portal imaging systems using this technique are also discussed.…”

Section: Introductionmentioning

confidence: 99%

Fast 2D phantom dosimetry for scanning proton beams

et al. 1998

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A quality control system especially designed for dosimetry in scanning proton beams has been designed and tested. The system consists of a scintillating screen (Gd2O2S:Tb), mounted at the beam-exit side of a phantom, and observed by a low noise CCD camera with a long integration time. The purpose of the instrument is to make a fast and accurate two-dimensional image of the dose distribution at the screen position in the phantom. The linearity of the signal with the dose, the noise in the signal, the influence of the ionization density on the signal, and the influence of the field size on the signal have been investigated. The spatial resolution is 1.3 mm (1 s.d.), which is sufficiently smaller than typical penumbras in dose distributions. The measured yield depends linearly on the dose and agrees within 5% with the calculations. In the images a signal to noise ration (signal/1 s.d.) of 10(2) has been found, which is in the same order of magnitude as expected from the calculations. At locations in the dose distribution possessing a strong contribution of high ionization densities (i.e., in the Bragg peak), we found some quenching of the light output, which can be described well by existing models if the beam characteristics are known. For clinically used beam characteristics such as a Spread Out Bragg peak, there is at most 8% deviation from the NACP ionization chamber measurements. The conclusion is that this instrument is a useful tool for quick and reliable quality control of proton beams. The long integration-time capabilities of the system make it worthwhile to investigate its applicability in scanning proton beams and other dynamic treatment modalities.

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Section: Introductionmentioning

confidence: 99%

Fast 2D phantom dosimetry for scanning proton beams

et al. 1998

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“…The dosimetric properties of some EPIDs have been investigated. [13][14][15][16][17][18] These detectors show promise in measuring the portal ''dose'' images ͑PDIs͒, although further investigation is necessary. For absolute dosimetry it is necessary to integrate the exit dose throughout the duration of the treatment beam.…”

Section: Introductionmentioning

confidence: 99%

Analysis and convergence of the iterative convolution/superposition dose reconstruction technique for multiple treatment beams and tomotherapy

1997

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An iterative convolution/superposition (C/S) algorithm has been created to reconstruct dose distributions in patients from exit dose measurements during a radiotherapy treatment. The method is based on an extended phantom which includes the patient CT representation and an electronic portal imaging device (EPID). The patient CT is assumed to be a true and rigid representation of the patient at the time of treatment. The C/S method computes the dose throughout the extended phantom which allows the exit dose to be predicted in the EPID. The process is then reversed to take the exit dose measurement and infer what the dose distribution must have been to produce the measured exit dose. The dose distribution is modeled without knowledge of the incident intensity distribution, and includes the effects of scatter in the computation. The iterative method begins by assuming that the exit primary energy fluence (PEF) is equal to the exit dose, the PEF is then backprojected through the extended phantom and superposed with the dose deposition kernel to determine a new prediction of the exit dose. The ratio of the computed PEF to exit dose is then multiplied by the measured exit dose image to produce a better representation of the exit PEF. Successive iterations then converge to the exit PEF image that would produce the measured exit dose image. Once convergence is established, the dose distribution is determined by backprojecting the exit PEF followed by superposition with the dose deposition kernel. The method is used to reconstruct the dose from a stimulated dynamic wedge and verified with film. Convergence and termination of the algorithm is then investigated with no noise and in the presence of noise. The method is then expanded to handle multiple treatment beams by separating the representation of the EPID from the patient or phantom representation in the computation process. Investigation of the effects of noise during the process of iterative dose reconstruction is necessary to understand the capabilities of the algorithm using exit dose images that may contain significant amounts of noise. The capability of the algorithm is evaluated for multiple field treatments to a cube phantom and a prostate patient CT representation in the presence of noise. The method is then used to simulate the dose reconstruction process for tomotherapy using 72 intensity-modulated fan beams. Dose reconstruction is shown to be capable of verifying the dose distributions in patients including multiple beams and dynamic collimation, provided the patient CT is known at the time of treatment.

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“…Efforts to determine and quantify ( in vivo ) dose in two and three dimensions are underway. The earliest works investigated the characteristics of the various EPIDs for transmission dose measurement 90 – 94 . These studies indicate that with the proper calibration and care, the EPID can be used to generate an exit dose image within 2–5 % of expected values.…”

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 device for exit dosimetry and quality control measurements

Cited by 91 publications

References 32 publications

Fast 2D phantom dosimetry for scanning proton beams

Fast 2D phantom dosimetry for scanning proton beams

Analysis and convergence of the iterative convolution/superposition dose reconstruction technique for multiple treatment beams and tomotherapy

Guide to clinical use of electronic portal imaging

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