Reducing the weight of the EC-L cassette

Munro, Peter; Mulligan, Michael J.

doi:10.1016/s0360-3016(02)02912-7

Cited by 3 publications

(1 citation statement)

References 21 publications

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“…Unfortunately many factors ͑e.g., field size, patient thickness, air-gap between patient and imager͒ effect the spectrum of photons reaching the detector. 46 If scatter cannot be neglected a scatter correction must be performed. [21][22][23] The assumption that the ADAS is more suited for exit dose measurements, because the build-up material is made of tissue-equivalent polyethylene, cannot be verified.…”

Section: Discussionmentioning

confidence: 99%

Performance of electronic portal imaging devices (EPIDs) used in radiotherapy: Image quality and dose measurements

Cremers¹,

Frenzel²,

Kausch³

et al. 2004

Medical Physics

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The aim of our study was to compare the image and dosimetric quality of two different imaging systems. The first one is a fluoroscopic electronic portal imaging device (first generation), while the second is based on an amorphous silicon flat-panel array (second generation). The parameters describing image quality include spatial resolution [modulation transfer function (MTF)], noise [noise power spectrum (NPS)], and signal-to-noise transfer [detective quantum efficiency (DQE)]. The dosimetric measurements were compared with ionization chamber as well as with film measurements. The response of the flat-panel imager and the fluoroscopic-optical device was determined performing a two-step Monte Carlo simulation. All measurements were performed in a 6 MV linear accelerator photon beam. The resolution (MTF) of the fluoroscopic device (f 1/2 = 0.3 mm(-1)) is larger than of the amorphous silicon based system (f 1/2 = 0.21 mm(-1)), which is due to the missing backscattered photons and the smaller pixel size. The noise measurements (NPS) show the correlation of neighboring pixels of the amorphous silicon electronic portal imaging device, whereas the NPS of the fluoroscopic system is frequency independent. At zero spatial frequency the DQE of the flat-panel imager has a value of 0.008 (0.8%). Due to the minor frequency dependency this device may be almost x-ray quantum limited. Monte Carlo simulations verified these characteristics. For the fluoroscopic imaging system the DQE at low frequencies is about 0.0008 (0.08%) and degrades with higher frequencies. Dose measurements with the flat-panel imager revealed that images can only be directly converted to portal dose images, if scatter can be neglected. Thus objects distant to the detector (e.g., inhomogeneous dose distribution generated by a modificator) can be verified dosimetrically, while objects close to a detector (e.g., a patient) cannot be verified directly and must be scatter corrected prior to verification. This is justified by the response of the flat-panel imaging device revealing a strong dependency at low energies.

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Section: Discussionmentioning

confidence: 99%

Performance of electronic portal imaging devices (EPIDs) used in radiotherapy: Image quality and dose measurements

Cremers¹,

Frenzel²,

Kausch³

et al. 2004

Medical Physics

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Monte Carlo simulation of portal dosimetry on a rectilinear voxel geometry: a variable gantry angle solution

Chin

Spezi²,

Lewis³

2003

Phys. Med. Biol.

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A software solution has been developed to carry out Monte Carlo simulations of portal dosimetry using the BEAMnrc/DOSXYZnrc code at oblique gantry angles. The solution is based on an integrated phantom, whereby the effect of incident beam obliquity was included using geometric transformations. Geometric transformations are accurate within +/- 1 mm and +/- 1 degrees with respect to exact values calculated using trigonometry. An application in portal image prediction of an inhomogeneous phantom demonstrated good agreement with measured data, where the root-mean-square of the difference was under 2% within the field. Thus, we achieved a dose model framework capable of handling arbitrary gantry angles, voxel-by-voxel phantom description and realistic particle transport throughout the geometry.

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Monte Carlo portal dosimetry

Chin¹

2005

Medical Physics

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This project developed a solution for verifying external photon beam radiotherapy. The solution is based on a calibration chain for deriving portal dose maps from acquired portal images, and a calculation framework for predicting portal dose maps. Quantitative comparison between acquired and predicted portal dose maps accomplishes both geometric (patient positioning with respect to the beam) and dosimetric (two‐dimensional fluence distribution of the beam) verifications. A disagreement would indicate that beam delivery had not been according to plan. The solution addresses the clinical need for verifying radiotherapy both pretreatment (without the patient in the beam) and on treatment (with the patient in the beam). Medical linear accelerators mounted with electronic portal imaging devices (EPIDs) were used to acquire portal images. Two types of EPIDs were investigated: the amorphous silicon and the scanning liquid ion chamber (SLIC). The EGSnrc family of Monte Carlo codes were used to predict portal dose maps by computer simulation of radiation transport in the beam‐phantom‐EPID configuration. Monte Carlo simulations have been implemented on several levels of high throughput computing (HTC), including the grid, to reduce computation time. The solution has been tested across the entire clinical range of gantry angle, beam size , and beam‐patient and patient‐EPID separations . In these tests of known beam‐phantom‐EPID configurations, agreement between acquired and predicted portal dose profiles was consistently within 2% of the central axis value. This Monte Carlo portal dosimetry solution therefore achieved combined versatility, accuracy, and speed not readily achievable by other techniques.

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Reducing the weight of the EC-L cassette

Cited by 3 publications

References 21 publications

Performance of electronic portal imaging devices (EPIDs) used in radiotherapy: Image quality and dose measurements

Performance of electronic portal imaging devices (EPIDs) used in radiotherapy: Image quality and dose measurements

Monte Carlo simulation of portal dosimetry on a rectilinear voxel geometry: a variable gantry angle solution

Monte Carlo portal dosimetry

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