Simple Proposal for Dosimetry with an Elekta iViewGTTM Electronic Portal Imaging Device (EPID) Using Commercial Software Modules

Image quality improvements of electronic portal imaging devices by multi-level gain calibration and temperature correction

Amorphous silicon (aSi:H) flat panel detectors are prevalent in radiotherapy for megavoltage imaging tasks. Any clinical and dosimetrical application requires a well-defined dose response of the system to achieve meaningful results. Due to radiation damages, panels deteriorate and the linearity of pixel response to dose as well as the stability with regard to changing operating temperatures get worse with time. Using a single level gain correction can lead to an error of about 23% when irradiating a flood field image with 100 MU min(-1) on an old detector. A multi-level gain (MLG) correction is introduced, emending the nonlinearities and subpanel-related artifacts caused by insufficient radiation hardness of amplifiers in the read-out electronics. With rising temperature, offset values typically increase (up to 300 gray values) while the response at higher dose values per frame remain constant for a majority of pixels. To account for temperature-related image artifacts, two additional temperature correction methods have been developed. MLG in combination with temperature corrections can re-establish the aSi:H image quality to the performance required by reliable medical verification tools. Furthermore, the life span and recalibration intervals of these costly devices can be prolonged decisively.

Analysis of a free-running synchronization artifact correction for MV-imaging with aSi:H flat panels

PURPOSE

Solid state flat panel electronic portal imaging devices (EPIDs) are widely used for megavolt (MV) photon imaging applications in radiotherapy. In addition to their original purpose in patient position verification, they are convenient to use in quality assurance and dosimetry to verify beam geometry and dose deposition or to perform linear accelerator (linac) calibration procedures. However, native image frames from amorphous silicon (aSi:H) detectors show a range of artifacts which have to be eliminated by proper correction algorithms. When a panel is operated in free-running frame acquisition mode, moving vertical stripes (periodic synchronization artifacts) are a disturbing feature in image frames. Especially for applications in volumetric intensity modulated arc therapy (VMAT) or motion tracking, the synchronization (sync) artifacts are the limiting factor for potential and accuracy since they become even worse at higher frame rates and at lower dose rates, i.e., linac pulse repetition frequencies (PRFs).

METHODS

The authors introduced a synchronization correction method which is based on a theoretical model describing the interferences of the panel's readout clocking with the linac's dose pulsing. Depending on the applied PRF, a certain number of dose pulses is captured per frame which is readout columnwise, sequentially. The interference of the PRF with the panel readout is responsible for the period and the different gray value levels of the sync stripes, which can be calculated analytically. Sync artifacts can then be eliminated multiplicatively in precorrected frames without additional information about radiation pulse timing.

RESULTS

For the analysis, three aSi:H EPIDs of various types were investigated with 6 and 15 MV photon beams at varying PRFs of 25, 50, 100, 200, and 400 pulses per second. Applying the sync correction at panels with gadolinium oxysulfide scintillators improved single frame flood field image quality drastically [improvement of the signal-to-noise ratio (SNR) up to 66.1 dB for 6 MV and 66.0 dB for 15 MV]. Also for the EPID with a caesium iodide scintillator, the noise for the lower PRFs could be reduced (SNR at 6 MV of up to 56.3 dB and at 15 MV up to 46.7 dB). However, the simplistic readout interference model fails at higher PRFs, where image lag and ghosting effects due to trapped charges in the thin film transistor and scintillator postglowing require additional corrections.

CONCLUSIONS

The presented free-running sync correction method improves SNR of single frames and enables imaging applications, like low-dose rate imaging at increased image frame rates (e.g., to track moving gold fiducials in the lung). Adaptive image guided radiotherapy protocols become even feasible in VMAT plans. Also simultaneous kilovolt and MV imaging applications can benefit from new possibilities of MV scatter removal in x-ray images.

Feasibility study of patient positioning verification in electron beam radiotherapy with an electronic portal imaging device (EPID)

The purpose of this study is to demonstrate the feasibility of verification and documentation in electron beam radiotherapy using the photon contamination detected with an electronic portal imaging device. For investigation of electron beam verification with an EPID, the portal images are acquired irradiating two different tissue equivalent phantoms at different electron energies. Measurements were performed on an Elekta SL 25 linear accelerator with an amorphous-Si electronic portal imaging device (EPID: iViewGT, Elekta Oncology Systems, Crawley, UK). As a measure of EPID image quality contrast (CR) and signal-to-noise ratio (SNR) are determined. For characterisation of the imaging of the EPID RW3 slabs and a Gammex 467 phantom with different material inserts are used. With increasing electron energy the intensity of photon contamination increases, yielding an increasing signal-to-noise ratio, but images are showing a decreasing contrast. As the signal-to-noise ratio saturates with increasing dose a minimum of 50 MUs is recommended. Even image quality depends on electron energy and diameter of the patient, the acquired results are mostly sufficient to assess the accuracy of beam positioning. In general, the online EPID acquisition has been demonstrated to be an effective electron beam verification and documentation method. The results are showing that this procedure can be recommended to be routinely and reliably done in patient treatment with electron beams.

Three-dimensional surface scanning for accurate patient positioning and monitoring during breast cancer radiotherapy

PURPOSE

Clinical evaluation of an optical three-dimensional surface scanning (3D-SS) system for patient positioning and monitoring during radiotherapy (RT) for breast cancer.

MATERIALS AND METHODS

A ceiling-mounted scanner was developed to acquire multiple 3D body surface images and tested in 14 conservatively operated breast cancer patients. A reference skin surface was derived from the planning computed tomography (CT) scan as basis for rigid registration with the surface scans. In addition to electronic portal images (EPIs), optical scans were acquired at three defined time points before and during daily RT. Patient setup was guided by laser alignments and corrected according to EPI findings. The accuracy of the 3D-SS system was validated by comparison of the optical scans to EPIs generated in parallel. Interfraction shifts were investigated by comparison of the first 3D-SS image with the reference body outline. Intrafractional motions were analysed by comparing the three daily surface scans with the first EPI.

RESULTS

Comparison of EPIs and 3D-SS images revealed good accordance (- 0.05±0.94 mm). Analysis of daily patient positions revealed average deviations of 0.4±2.4 mm laterally, 0.3±1.9 mm longitudinally and 0.2±3.3 mm vertically. After 2 weeks, a systematic interfraction shift in patient positioning was noted, particularly in the vertical direction (4.9±0.56 mm), which was attributed to patients progressively relaxing. 3D-SS images showed intrafractional shifts of 1.2±0.7 mm over a time course of 2 min.

CONCLUSION

Optical surface scanning is a simple, fast and reproducible method for breast cancer patient alignment. Particularly for more sophisticated irradiation techniques, it helps to improve accuracy in patient positioning during radiotherapy without the exposure to additional ionizing radiation.

Phase-specific cone beam computed tomography reduces reconstructed volume loss of moving phantom

PURPOSE

The accurate volumetric calculation of moving targets/organs is required to use cone-beam computed tomography (CBCT) for replanning purposes. This study was aimed to correct the reconstructed volume losses of moving phantoms by phase-specific CBCT.

MATERIALS AND METHODS

Planning fan-beam CT (FBCT) of five hepatobiliary/gastrointestinal/pancreatic cancer patients were acquired under active breathing control and compared with free-breathing CBCT for kidney volumes. Three different-sized ball phantoms were scanned by FBCT and CBCT. Images were imported to a planning system to compare the reconstructed volumes. The phantoms were moved longitudinally on an oscillator with different amplitudes/frequencies. The phase-specific projections of CBCT for moving phantoms were selected for volume reconstruction.

RESULTS

The differences in reconstructed volumes of static small, medium, large phantoms between FBCT and CBCT were - 6.7%, - 2.3%, and - 2.0%, respectively. With amplitudes of 7.5-20 mm and frequencies of 8-16 oscillations/min, volume losses on CBCT were comparable with FBCT in large moving phantoms (range 9.1-27.2%). Amplitudes were more subject to volume losses than frequencies. On phase-specific CBCT, volume losses were reduced to 2.3-6.5% by reconstruction using 2-3 projections at end/midoscillation phase.

CONCLUSION

Amplitude had more impact than frequency on volume losses of moving phantoms on CBCT. Phase-specific CBCT reduced volume losses.