Conventional Electronic Portal Imaging Devices Research Articles

Beam's-Eye-View Imaging of Liver SBRT With a Novel Multi-Layer Imager

Beams-eye view (BEV) imaging during radiation therapy can be used for real-time localization of the treatment target. However, clinical implementation is limited by the poor performance of current electronic portal imaging devices (EPIDs). A novel multilayer imager (MLI), consisting of four stacked conventional flat-panel imagers, was recently shown to have 5.7 times the detective quantum efficiency and nearly 2 times the contrast-to-noise ratio of a conventional EPID. In the current study, we compare the MLI against a single layer imager for the application of fiducial tracking during liver SBRT procedures. The hypothesis is that the MLI will provide more accurate and efficient fiducial tracking.The prototype MLI was installed in the standard MV imager housing on a clinical LINAC, replacing the conventional EPID. The MV imager arm was extended during volumetric modulated arc therapy (VMAT) SBRT treatments for passive data acquisition. Six patients with liver metastases receiving SBRT in 3-5 fractions (900-1200 cGy/fx) were studied. Data was acquired for two fractions of each treatment course, one fraction with the MLI using all 4 layers and one with the MLI using the top layer only. Automatic fiducial localization was performed using a modification of a previously published algorithm. Truth data was derived using external surrogate respiratory traces combined with manual tracking. Results for 4- and 1-layer modes were compared against truth data to determine tracking accuracy and efficiency. Tracking and noise improvements were assessed for any correlation.Between 2771 to 6505 image frames were acquired for each patient, with roughly half of each corresponding to acquisition in 1-layer mode and the other half 4-layer mode. Tracking accuracy with the 1-layer imager returned a root mean square error (RMSE) of 2.1 mm compared to 4-layer RMSE of 1.5 mm, a statistically significant (P < 0.001) improvement of 0.6 mm. Successful tracking efficiency with 4 layers improved to 82.8% versus 58.4% for the 1-layer mode, a relative improvement of 41.8% (P < 0.001). There was a strong correlation (r = 0.913) between the increase in successfully tracked frames and reduction in noise when using the 4-layer mode.Noise reduction and increased MV photon detection efficiency achieved by utilizing a multi-layer MV imager results in improved fiducial tracking for liver SBRT treatments. Future BEV clinical applications may be improved by pursuing similar noise reduction and photon detection enhancement strategies.

Characterization of Gas Electron Multiplier‐based detector for external beam radiation therapy dosimetry

Electronic portal imaging devices (EPIDs) are commonly installed on modern linear accelerators (LINACs) and are convenient for imaging and, potentially, dosimetry. However, owing to their construction with metal and scintillating layers of high atomic number, they exhibit nonwater-equivalent response and oversensitivity to low-energy photons. Therefore, EPIDs are not ideal for dosimetry purposes. Additionally, nonlinearities due to the combined use of scintillators and photodiodes have been reported. Here, an EPID which employs a variable gain Gas Electron Multiplier (GEM) and direct detection of electrons is introduced. To investigate its dosimetric performance, measurements characterizing the novel EPID are performed and compared with measurements from ionization chambers and conventional EPIDs. Linearity, dose rate dependence, field size dependence, off-axis response, and transmission response were measured for all available energy settings (6, 10, 6MV Flattening Filter Free (FFF) and 10MVFFF) using three different detector gain settings. Additionally, an evaluation of the ghosting and image lag of the panel was completed. Reference ionization chamber measurements were performed for the off-axis and transmission response and existing data for conventional EPIDs and ionization chambers from equivalent measurements were used for comparison of the field size dependence. Elsewhere, values from the linac monitoring chambers were used. In the range from 10 to 1000 Monitor Units (MU), the detector was linear within 1% for all combinations of gain settings and energies. The dose rate dependence was also within 1% for all energies and for two out of three gain settings. Regarding field size dependency, the ratio of ionization chamber and panel values was 0.94 and 0.98 for the conventional EPID and GEMini respectively, at 20×20cm2 and 10MV. For 6MV, 6MVFFF, and 10MVFFF these ratios were 0.97, 0.98, and 0.99 for the GEMini, and 0.95, 0.97, and 0.97 for the conventional EPID. Similar performance between the GEMini and conventional EPID is observed for field sizes smaller than 10×10cm2 . The transmission response was within 5% for all energies for thicknesses up to 30cm, compared to 10-20% for a conventional EPID. The off-axis response for shifts up to 16cm was within 1% and 3% for 6MV and 10MV, with and without phantom. The rise and fall of the signal from the detector correspond well to monitor chamber measurements indicating little ghosting and image lag, regardless of gain setting. The GEM EPID exhibits dose rate dependence and linearity within 1%, and negligible ghosting and image lag. In this regard, it performs particularly well using 50 and 250V of gain, and either could be chosen. For higher sensitivity, 250V is the recommended base gain setting, although other applications may warrant different gains. For most tests performed in this study, the GEM EPID demonstrates a more water-equivalent response than conventional EPIDs making GEMs a viable technology for dosimetry in radiation therapy.

Clinical translation of a new flat-panel detector for beam’s-eye-view imaging

Electronic portal imaging devices (EPIDs) lend themselves to beams-eye view clinical applications, such as tumor tracking, but are limited by low contrast and detective quantum efficiency (DQE). We characterize a novel EPID prototype consisting of multiple layers and investigate its suitability for use under clinical conditions. A prototype multi-layer imager (MLI) was constructed utilizing four conventional EPID layers, each consisting of a copper plate, a Gd2O2S:Tb phosphor scintillator, and an amorphous silicon flat panel array detector. We measured the detector’s response to a 6 MV photon beam with regards to modulation transfer function, noise power spectrum, DQE, contrast-to-noise ratio (CNR), signal-to-noise ratio (SNR), and the linearity of the detector’s response to dose. Additionally, we compared MLI performance to the single top layer of the MLI and the standard Varian AS-1200 detector. Pre-clinical imaging was done on an anthropomorphic phantom, and the detector’s CNR, SNR and spatial resolution were assessed in a clinical environment. Images obtained from spine and liver patient treatment deliveries were analyzed to verify CNR and SNR improvements. The MLI has a DQE(0) of 9.7%, about 5.7 times the reference AS-1200 detector. Improved noise performance largely drives the increase. CNR and SNR of clinical images improved three-fold compared to reference. A novel MLI was characterized and prepared for clinical translation. The MLI substantially improved DQE and CNR performance while maintaining the same resolution. Pre-clinical tests on an anthropomorphic phantom demonstrated improved performance as predicted theoretically. Preliminary patient data were analyzed, confirming improved CNR and SNR. Clinical applications are anticipated to include more accurate soft tissue tracking.

Technical Note: EPID's response to 6MV photons in a strong, parallel magnetic field.

Electronic portal imaging devices (EPIDs) are potentially useful for dosimetric verification in integrated MRI-linac systems. This work presents the reproducibility, linearity, image lag, and radiation field profiles in a conventional EPID, with and without a 0.5T parallel magnetic field present in a 6MV photon beam. An aS500 EPID was modified to function in strong magnetic fields. All measurements were made using the linac-MR installed at the Cross Cancer Institute. The EPID remained stationary on the couch between the measurements made with and without magnetic field. We measured short-term reproducibility of dark and flood fields, signal linearity from 1 to 500MU irradiations, and image lag post 100MU irradiation. An ion chamber was used to measure any linac output variations to correct the EPID signal due to these variations for the duration of experiment. X-axis and Y-axis radiation field profiles were obtained from the EPID image resulting from a 10×10cm2 radiation field delivery. The average pixel value (±standard deviation) of flood field with and without magnetic fields were 57,876±379 and 57,703±366, respectively, and the corresponding average dark field pixel values were -32.05±0.85 and -32.19±0.97. The maximum difference in image linearity data with and without magnetic field is 0.2% which is well within the measurement uncertainty of 0.65%. Similarly, the image lag curves, with and without the magnetic field, were nearly identical. The first measured point, with mean lag signal of 1.44% without and 1.41% with magnetic field, shows that the largest difference is well below the uncertainty in the EPID signal measurement. The radiation field profiles obtained with and without magnetic fields were nearly identical; 91.3% of the X-axis and 95.2% of the Y-axis profile points pass a gamma criterion of 1% and 1mm. A conventional EPID imager with a 0.1cm copper plate responds to 6MV photons similarly irrespective of the strong magnetic field being off or on in the parallel orientation to the radiation beam. Thus, the EPID is a potentially useful tool for pretreatment dosimetric verification in linac-MR systems using parallel magnetic field.

Technical Note: Combination of multiple EPID imager layers improves image quality and tracking performance of low contrast‐to‐noise objects

We hypothesized that combining multiple amorphous silicon flat panel layers increases photon detection efficiency in an electronic portal imaging device (EPID), improving image quality and tracking accuracy of low-contrast targets during radiotherapy. The prototype imager evaluated in this study contained four individually programmable layers each with a copper converter layer, Gd2 O2 S scintillator, and active-matrix flat panel imager (AMFPI). The imager was placed on a Varian TrueBeam linac and a Las Vegas phantom programmed with sinusoidal motion (peak-to-peak amplitude = 20 mm, period = 3.5 s) was imaged at a frame rate of 10 Hz with one to four layers activated. Number of visible circles and CNR of least visible circle (depth = 0.5 mm, diameter = 7 mm) was computed to assess the image quality of single and multiple layers. A previously validated tracking algorithm was employed for auto-tracking. Tracking error was defined as the difference between the programmed and tracked positions of the circle. Pearson correlation coefficient (R) of CNR and tracking errors was computed. Motion-induced blurring significantly reduced circle visibility. During four cycles of phantom motion, the number of visible circles varied from 11-23, 13-24, 15-25, and 16-26 for one-, two-, three-, and four-layer imagers, respectively. Compared with using only a single layer, combining two, three, and four layers increased the median CNR by factors of 1.19, 1.42, and 1.71, respectively and reduced the average tracking error from 3.32 mm to 1.67 mm to 1.47 mm, and 0.74 mm, respectively. Significant correlations (P~10-9 ) were found between the tracking error and CNR. Combination of four conventional EPID layers significantly improves the EPID image quality and tracking accuracy for a poorly visible object which is moving with a frequency and amplitude similar to respiratory motion.

Super-resolution imaging in a multiple layer EPID

A new portal imager consisting of four vertically stacked conventional electronic portal imaging device layers has been constructed in pursuit of improved detective quantum efficiency. We hypothesize that super-resolution (SR) imaging can also be achieved in such a system by shifting each layer laterally by half a pixel relative to the layer above. SR imaging will improve resolution and contrast-to-noise ratio (CNR) in megavoltage (MV) planar and cone beam computed tomography (MV-CBCT) applications. Simulations are carried out to test this hypothesis with digital phantoms. To assess planar resolution, 2 mm long iron rods with 0.3 × 0.3 mm2 square cross-section are arranged in a grid pattern at the center of a 1 cm thick solid water. For measuring CNR in MV-CBCT, a 20 cm diameter digital phantom with 8 inserts of different electron densities is used. For measuring resolution in MV-CBCT, a digital phantom featuring a bar pattern similar to the GammexTM phantom is used. A 6 MV beam is attenuated through each phantom and detected by each of the four detector layers. Fill factor of the detector is explicitly considered. Projections are blurred with an estimated point spread function before SR reconstruction. When projections from multiple shifted layers are used in SR reconstruction, even a simple shift-add fusion can significantly improve the resolution in reconstructed images. In the reconstructed planar image, the grid pattern becomes visually clearer. In MV-CBCT, combining projections from multiple layers results in increased CNR and resolution. The inclusion of two, three and four layers increases CNR by 40%, 70% and 99%, respectively. Shifting adjacent layers by half a pixel almost doubles resolution. In comparison, using four perfectly aligned layers does not improve resolution relative to a single layer.

A novel EPID design for enhanced contrast and detective quantum efficiency

Beams-eye-view imaging applications such as real-time soft-tissue motion estimation are hindered by the inherently low image contrast of electronic portal imaging devices (EPID) currently available for clinical use. We introduce and characterize a novel EPID design that provides substantially increased detective quantum efficiency (DQE), contrast-to-noise ratio (CNR) and sensitivity without degradation in spatial resolution.The prototype design features a stack of four conventional EPID layers combined with low noise integrated readout electronics. Each layer consists of a copper plate, a scintillator () and a photodiode/TFT-switch (aSi:H). We characterize the prototype’s signal response to a 6 MV photon beam in terms of modulation transfer function (MTF), DQE and CNR. The presampled MTF is estimated using a slanted slit technique, the DQE is calculated from measured normalized noise power spectra (nNPS) and the MTF and CNR is estimated using a Las Vegas contrast phantom. The prototype has been designed and built to be interchangeable with the current clinical EPID on the Varian TrueBeam platform (AS-1200) in terms of size and data output specifications. Performance evaluation is conducted in absolute values as well as in relative terms using the Varian AS-1200 EPID as a reference detector.A fivefold increase of DQE(0) to about 6.7% was observed by using the four-layered design versus the AS-1200 reference detector. No substantial differences are observed between each layer’s individual MTF and the one for all four layers operating combined indicating that defocusing due to beam divergence is negligible. Also, using four layers instead of one increases the signal to noise ratio by a factor of 1.7.

WE‐DE‐BRA‐07: Megavoltage Spectral Imaging with a Layered Detector

Purpose:The aim of the current work is to investigate the feasibility of megavoltage spectral imaging using a multiple layered detector for enhancement of low contrast detectability through material segmentation and discrimination (such as bone, markers and metal implants). Potentially the technique can be applied to improve detection and reduce dose in Megavoltage Cone Beam Computed Tomography (MV‐CBCT).Methods:Experiments were performed with a prototype multi‐layer imager (MLI) which has higher detective efficiency and lower noise characteristics than conventional Electronic Portal Imaging Devices (EPIDs). Images of a solid water phantom were acquired at 2.5 MV, 6MV and 6MV without flattening filter (FFF). The following materials were placed within a stack of solid water: aluminum, copper and gold. Material separation was assessed based on Contrast‐to‐Noise Ratio (CNR) of the weighted image, formed by a weighted subtraction of the images from two layers of the MLI. A range of weighting factors were investigated for material separation.Results:CNR can be minimized for each material by appropriate selection of the subtraction weighting factor. This is equivalent to a selective subtraction of specific materials from the image. Using multiple layers simultaneously also decreases the dose requirement and removes any registration errors. The minimum CNR for aluminum, copper and gold at the weighted image formed with 2.5MV was obtained at weighting factors equal to 0.92, 0.76 and 0.64 respectively. The corresponding values at 6MVFFF were 0.99, 0.92 and 0.78 respectively.Conclusion:In the current work, an MV spectral imaging feasibility study was attempted using a novel multi‐layer prototype EPID imager. Initial results suggest that material separation based on spectral differences between different layers is possible. This spectral imaging technique has potential advantages in MV‐CBCT for real‐time target tracking, patient set‐up imaging and adaptive radiotherapy.The project was supported, partially, by a grant from Varian Medical Systems, Inc., and Award No. R01CA188446‐01 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health

SU‐C‐201‐05: Silicon Array Dosimeter in Situ with Electronic Portal Image Device for Simultaneous Transit Dose and Image Verification in Radiotherapy

Purpose:To investigate an Electronic Portal Imaging Device (EPID) coupled to a 2D array dosimeter to provide simultaneous imaging and dose verification.Methods:The novel dual detector configuration comprised of a 2D diode array dosimeter, referred to as a Magic Plate (MP) placed directly on a standard EPID. Dose response of the MP was evaluated by measuring the detector's response with respect to off‐axis position and field size with 30 cm of solid water (SW) acting as a transit object in the beam. Measurements were performed with 3, 5, 10 and 15 mm SW build‐up and compared to 2D ionisation chamber array (ICA) measurements and the PinnacleTM treatment planning system (TPS) at a source to detector distance of 150 cm with a 6 MV beam. Clinical dosimetric performance was evaluated by measuring a number of intensity‐modulated radiation therapy (IMRT) beams in transit geometry. Imaging performance of the EPID was quantified by measuring the contrast‐to‐noise ratio (CNR) and spatial resolution. Images of a Rando phantom were used for qualitative assessment.Results:Measured MP off‐axis and field size response agreed within 2% of TPS and ICA responses when measured using 15 mm SW build‐up. Clinical IMRT beams had gamma pass rates of ≥95% at 3%/3mm criteria. Measured CNR and spatial resolution (f50) were 264.96, 210.6, and 0.41, 0.40 with build‐up of 5 and 15 mm respectively for the dual detector configuration. CNR and spatial resolution of 643.9 and 0.41 were measured for standard EPID. CNR was quantitatively worse in the dual detector configuration. Differences in imaging performance were not visible in a qualitative assessment using a Rando phantom.Conclusion:Combining a prototype MP 2D dosimeter with a conventional EPID did not significantly detract from the performance of either device and has the potential for simultaneous on‐line patient transit dosimetry and image assessment in radiation therapy.Cancer Institute NSW Australia(Research Equipment Grant 10/REG/1‐20) and Cancer Council NSW (Grant ID RG 1‐06)

TH‐EF‐BRB‐01: JUNIOR INVESTIGATOR WINNER — Novel EPID for Enhanced Contrast and Detective Quantum Efficiency

Purpose:Beams‐eye‐view imaging applications such as real‐time soft‐tissue motion estimation are hindered by the inherently low image contrast of electronic portal imaging devices (EPID) currently in clinical use. We introduce and characterize a novel EPID that provides substantially increased detective quantum efficiency (DQE), contrast‐to‐noise ratio (CNR) and dynamic range without degradation in spatial resolution.Methods:The prototype design features a stack of four conventional EPID layers combined with low noise integrated readout electronics. Each layer consists of a copper plate, a scintillator (GdO2S2:Tb) and a photodiode/TFT‐switch (aSi:H). We characterize the prototype in terms of contrast‐to‐noise ratio (CNR), modulation transfer function (MTF) and DQE. CNR is estimated using a Las Vegas contrast phantom, presampled MTF is estimated using a slanted edge technique and the DQE is calculated from measured normalized noise power spectra (NPS) and the MTF. The prototype has been designed and built to be interchangeable with the current clinical EPID in terms of size and data output specifications.Results:DQE(0) can be nearly quadrupled to about 4.5% by using the four‐layered design instead of only a single layer device. No substantial differences are observed between each layer's individual MTF and the one for all four layers operating combined indicating that defocusing is negligible. Also, using four layers instead of one nearly doubles (factor x1.9) the CNR showing that the device is mostly quantum noise limited.Conclusion:A layered EPID design allows improving the radiation sensitivity while maintaining spatial resolution and saturation level of a single layer conventional EPID. Experimental characterization of this first 4‐layered prototype demonstrates substantially improved DQE and CNR while maintaining high resolution and remaining quantum noise limited. Besides overall improved image quality and dosimetric sensitivity we anticipate this novel detector design to enable more accurate soft‐tissue motion estimations during radiation therapy procedures, particularly of the lung.The project was partially supported by a grant from Varian Medical Systems, Inc. and grant No. R01CA188446‐01 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.

TH‐EF‐BRB‐02: Combination of Multiple EPID Imager Layers Improves Image Quality and Tracking Performance of Low Contrast Objects

Purpose:We hypothesize that an electronic portal imaging device (EPID) consisting of multiple layers, each containing a scintillator coupled to amorphous silicon flat panel imager (AMFPI), increases photon detection efficiency, improving image quality and tracking accuracy of low contrast targets during radiotherapy.Methods:The prototype 4‐layer imager evaluated in this study contained four individually programmable layers each with a copper‐Gd₂O₂S scintillator coupled to an AMFPI with a complete set of readout electronics. The imager was placed on a Varian TrueBeam linac. A Las‐Vegas phantom programmed with sinusoidal motion (Peak‐to‐peak amplitude=20cm, Period=3.5sec) was imaged at a frame rate of 10Hz with one to four layers activated (activation of only the top layer is equivalent to using a conventional EPID). Image quality was assessed by counting the number of visible circles, and computing the contrast‐to‐noise ratio (CNR) of the least visible circle (depth=0.5mm, diameter=7mm). A previously validated tracking algorithm was employed for auto‐tracking. Tracking error was defined as the difference between the programmed and tracked positions of the circle. The Pearson correlation coefficient of the CNR and tracking error was also computed.Results:Motion‐induced EPID image blurring significantly reduced circle visibility. During one cycle of phantom motion, the number of visible circles ranged from 11–23, 13–24, 15–25, and 16–26 when imaged with one, two, three and four layer imagers, respectively. Compared with using only a single layer, combining 2, 3, and 4 layers increased the median CNR by factors of 1.19, 1.42, and 1.71, respectively and reduced the average tracking error from 3.32mm to 1.67mm to 1.47mm, and 0.74mm, respectively. Significant correlations (p∼10−9) were found between the tracking error and CNR.Conclusion:Combination of four amorphous silicon layers, instead of single layer, significantly improves the EPID image quality and tracking accuracy for a poorly visible object that replicates lung tumor radiotherapy conditions.The project described was supported, in part, by a grant from Varian Medical Systems, Inc., and Award No. R01CA188446‐01 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.

Feasibility study of a dual detector configuration concept for simultaneous megavoltage imaging and dose verification in radiotherapy

To test the feasibility of a dual detector concept for comprehensive verification of external beam radiotherapy. Specifically, the authors test the hypothesis that a portal imaging device coupled to a 2D dosimeter provides a system capable of simultaneous imaging and dose verification, and that the presence of each device does not significantly detract from the performance of the other. The dual detector configuration comprised of a standard radiotherapy electronic portal imaging device (EPID) positioned directly on top of an ionization-chamber array (ICA) with 2 cm solid water buildup material (between EPID and ICA) and 5 cm solid backscatter material. The dose response characteristics of the ICA and the imaging performance of the EPID in the dual detector configuration were compared to the performance in their respective reference clinical configurations. The reference clinical configurations were 6 cm solid water buildup material, an ICA, and 5 cm solid water backscatter material as the reference dosimetry configuration, and an EPID with no additional buildup or solid backscatter material as the reference imaging configuration. The dose response of the ICA was evaluated by measuring the detector's response with respect to off-axis position, field size, and transit object thickness. Clinical dosimetry performance was evaluated by measuring a range of clinical intensity-modulated radiation therapy (IMRT) beams in transit and nontransit geometries. The imaging performance of the EPID was evaluated quantitatively by measuring the contrast-to-noise ratio (CNR) and spatial resolution. Images of an anthropomorphic phantom were also used for qualitative assessment. The measured off-axis and field size response with the ICA in both transit and nontransit geometries for both dual detector configuration and reference dosimetry configuration agreed to within 1%. Transit dose response as a function of object thickness agreed to within 0.5%. All IMRT test patterns and clinical IMRT beams had gamma pass rates of ≥98% at 2%/2 mm criteria. In terms of imaging performance, the measured CNR and spatial resolution (f50) were 263.23 ± 24.85 and 0.4025 ± 1.25 × 10(-3) for dual detector configuration and 324 ± 26.65 and 0.4141 ± 1.14 × 10(-3) for reference imaging configuration, respectively. The CNR and spatial resolution were quantitatively worse in the dual detector configuration due to the additional backscatter. The difference in imaging performance was not visible in qualitative assessment of phantom images. Combining a commercially available ICA dosimetry device with a conventional EPID did not significantly detract from the performance of either device. Further improvements in imaging performance may be achieved with an optimized design. This study demonstrates the feasibility of a dual detector concept for simultaneous imaging and dosimetry in radiation therapy.

An inherent anti-scatter detector for megavoltage x-ray imaging

Scattered x-rays are detrimental to the image quality of x-ray transmission radiography. Anti-scatter grids have been developed for kilovoltage (kV) x-ray imaging but are impractical to use for megavoltage (MV) x-ray imaging in radiation therapy. Our goal is to develop a new approach that uses an inherent anti-scatter detector for scatter reduction in MV x-ray imaging. A Monte Carlo simulation has been conducted to evaluate the response of a recently proposed Čerenkov electronic portal imaging device (CPID) to scattered x-rays. The proposed detector consists of a matrix of optical fibers aligned with the incident x-rays and coupled to an active matrix flat panel imager for image readout. The effects of scatter on the signal and noise of the CPID in comparison with those of conventional electronic portal imaging devices (EPIDs) have been investigated. It has been found that the CPID is ∼50% less sensitive to scattered x-rays than conventional EPIDs at 6 MV. The differential signal to noise ratio is also improved by up to 30% in the CPID. The results of our simulations have demonstrated that the recently proposed CPID system is an inherent anti-scatter detector, the first of this kind, for MV x-ray imaging.

TH‐B‐211‐02: Patient Organ Doses From Imaging in IGRT: KV Radiograph Vs. KV‐ CBCT Vs. MV Portal Image Vs. MV‐CBCT

Image‐guided radiation therapy (IGRT) plays an increasingly essential role in the accurate delivery of highly conformal dose to the target as it makes it possible to see the target and organs at risk before treatment while the patient is in the treatment position. X‐ray imaging devices, especially kilovoltage x‐ray imaging devices integrated in a radiation treatment delivery unit, are increasingly available in the clinics. The conventional electronic portal imaging devices (EPID) using megavoltage photon beams are still available and being used as well. X‐ray imaging procedures deliver additional radiation dose to patients and this dose may entail risk to patients. Knowledge of radiation dose resulting from x‐ray imaging and the techniques to reduce the imaging dose are important for clinicians in making informed decisions for treatment management regarding the benefit and risk to patients. This lecture will provide an overview of radiation doses to patients resulting from different imaging procedures and an update on the progress of AAPM TG‐180 report.Learning Objectives:1. Understand the different available image guidance modalities and devices used in IGRT;2. Understand the magnitude of the organ dose resulting from difference devices and acquisition procedures;3. Understand the variation of patient imaging dose distributions among different imaging procedures;4. Understand how patient dose distributions from an image procedure are calculated5. Understand the increased magnitude of the dose to bone as compared to soft tissue for kilovoltage x‐rays;6. Learn the techniques to reduce the imaging dose to patients and sensitive organs;7. Update on the progress of AAPM TG‐180 report: “Modeling and Accounting for the Imaging Guidance Radiation Doses to Radiotherapy Patients in Treatment Planning”

TH‐C‐BRA‐11: First Experiments of a Prototype Device for Simultaneous Imaging and Dose Verification in Radiotherapy

Purpose: Current model electronic portal imaging devices (EPIDs) used in radiotherapy are optimised for imaging but problematic for accurate dosimetry. The aim of this project is to develop a new EPID capable of simultaneous imaging and water equivalent dosimetry. This work reports our first experimental results. Methods: A prototype device based on a segmented plastic scintillator (SegmentedPS) was developed. The prototype device was tested in comparison with three other detector configurations, all utilising the same a‐Si photodiode array used in conventional EPIDs. The configurations tested were; i) standard indirect configuration with phosphor/copper (STANDARD), ii) direct configuration with 1.5 cm solid water build‐up (DIRECT), iii) 2 cm thick sheet of plastic scintillator (PSsheet), and iv) 1.5 cm thick prototype SegmentedPS. The sensitivity, dose response and image quality was assessed in each configuration. The dose response was assessed in terms of field size response and off‐axis response relative to reference dose in water measurements at the equivalent depth. The image quality was assessed using an image quality phantom. Results: The sensitivity of each device relative to the STANDARD configuration was 0.03(DIRECT), 0.63(PSsheet), and 0.35(SegmentedPS). The agreement in field size response relative to dose in water data for square field sizes 4 cm to 15 cm was within 4.8%(STANDARD), 0.9%(DIRECT), 22%(PSsheet), and 1.3%(SegmentedPS). The agreement in off‐axis ratios at 15 cm off‐axis, relative to dose in water data, was within 23%(STANARD), 4%(DIRECT), 5%(PSsheet), and 4%(SegmentedPS). Image quality parameters (f50/CNR) for each configuration were 0.41/993(STANDARD), 0.30/167(DIRECT), 0.22/125(PSsheet) and 0.23/214(SegmentedPS). Conclusions: First experiments with the prototype SegmentedPS EPID demonstrated the potential for simultaneous imaging and water equivalent dosimetry. Further design optimisation is required to approach the imaging performance of STANDARD a‐Si EPIDs, while maintaining water equivalent dose response. Cancer Council NSW Research Project Grant (RG 11‐06) Cancer Institute NSW Research Equipment Grant (10/REG/1‐20)

WE-E-201B-07: Megavoltage Cone-Beam Computed Tomography Using 2.5MV X-Rays

Purpose: To evaluate the feasibility of 2.5MV cone‐beam computed tomography (MVCB) for patient setup and verification. Method and Materials: A new linac (Varian Trilogy Mx) produces an unfiltered 2.5MV imaging beam (2xMVCB) in addition to the higher energy treatment beams. In conjunction with a standard electronic portal imaging device(EPID), 2xMVCB scans were acquired of CTcalibration and anthropomorphic phantoms. Approximately 500 projections were collected in each 360‐degree scan using a total of 7.5 – 50 monitor units (MU). Depending on total MU, scanning time was as low as 1.5 minutes. Intrinsic calibration corrections were applied to all raw projections prior to reconstruction. Reconstructed images were further processed to correct for imaging artifacts and compared to kilovoltage cone‐beam (KVCB) and 6MV MVCB scans (6xMVCB). Results: Preliminary results demonstrate that it is possible to obtain 3D images with adequate image quality using relatively low doses (7.5 MU, approximately 4 cGy to isocenter) which are comparable to either a pair of double‐exposed MV portal images or a KVCB. Although image quality is inferior to KVCB, 2xMVCB still permits assessment of bony alignment for patient setup and has better tissuecontrast than 6xMVCB. Soft tissue registration may also be possible for 2xMVCB in the thorax and other anatomical regions. These results provide evidence that 2xMVCB may be an efficient image guidance procedure for clinical routine. Conclusion: MVCB using 2.5MV x‐rays can provide volumetric images with adequate quality for patient setup verification. An improved EPID optimized for 2.5MV beam having ∼14 times higher quantum efficiency than conventional EPIDs is being tested. This will enable even lower imaging doses, and also improve imagecontrast in the abdominal and pelvic regions. Reconstruction algorithms optimized for 2.5MV are also under development. Acknowledgements: Research sponsored by Varian Medical Systems.

Dosimetric verification of IMAT delivery with a conventional EPID system and a commercial portal dose image prediction tool

The electronic portal imaging device (EPID) is a system for checking the patient setup; as a result of its integration with the linear accelerator and software customized for dosimetry, it is increasingly used for verification of the delivery of fixed-field intensity-modulated radiation therapy (IMRT). In order to extend such an approach to intensity-modulated arc therapy (IMAT), the combined use of an EPID system and a portal dose image prediction (PDIP) tool has been investigated. The dosimetric behavior of an EPID system, mechanically reinforced to maintain its positional stability during the accelerator gantry rotation, has been studied to assess its ability to measure portal dose distributions for IMAT treatment beams. In addition, the PDIP tool of a commercial treatment planning system, commonly used for static IMRT dosimetry, has been validated for simulating the PDIs of IMAT treatment fields. The method has been applied to the delivery verification of 23 treatment fields that were measured in their dual mode of IMRT and IMAT modalities. The EPID system has proved to be appropriate for measuring the PDIs of IMAT fields; additionally the PDIP tool was able to simulate these accurately. The results are quite similar to those obtained for static IMRT treatment verification, although it was necessary to investigate the dependence of the EPID signal and of the accelerator monitor chamber response on variable dose rate. Our initial tests indicate that the EPID system, together with the PDIP tool, is a suitable device for the verification of IMAT plan delivery; however, additional tests are necessary to confirm these results.

High-DQE EPIDs based on thick, segmented BGO and CsI:Tl scintillators: performance evaluation at extremely low dose.

Electronic portal imaging devices (EPIDs) based on active matrix, flat-panel imagers (AMFPIs) have become the gold standard for portal imaging and are currently being investigated for megavoltage cone-beam computed tomography (CBCT) and cone-beam digital tomosynthesis (CBDT). However, the practical realization of such volumetric imaging techniques is constrained by the relatively low detective quantum efficiency (DQE) of AMFPI-based EPIDs at radiotherapy energies, approximately 1% at 6 MV. In order to significantly improve DQE, the authors are investigating thick, segmented scintillators, consisting of 2D matrices of scintillating crystals separated by septal walls. A newly constructed segmented BGO scintillator (11.3 mm thick) and three segmented CsI:Tl scintillators (11.4, 25.6, and 40.0 mm thick) were evaluated using a 6 MV photon beam. X-ray sensitivity, modulation transfer function, noise power spectrum, DQE, and phantom images were obtained using prototype EPIDs based on the four scintillators. The BGO and CsI:Tl prototypes were found to exhibit improvement in DQE ranging from approximately 12 to 25 times that of a conventional AMFPI-based EPID at zero spatial frequency. All four prototype EPIDs provide significantly improved contrast resolution at extremely low doses, extending down to a single beam pulse. In particular, the BGO prototype provides contrast resolution comparable to that of the conventional EPID, but at 20 times less dose, with spatial resolution sufficient for identifying the boundaries of low-contrast objects. For this prototype, however, the BGO scintillator exhibited an undesirable radiation-induced variation in x-ray sensitivity. Prototype EPIDs based on thick, segmented BGO and CsI:T1 scintillators provide significantly improved portal imaging performance at extremely low dose (i.e., down to 1 beam pulse corresponding to approximately 0.022 cGy), creating the possibility of soft-tissue visualization using MV CBCT and CBDT at clinically practical dose.

Clinical Feasibility of Using an EPID in cine Mode for Image-Guided Verification of Stereotactic Body Radiotherapy

To introduce a novel method for monitoring tumor location during stereotactic body radiotherapy (SBRT) while the treatment beam is on by using a conventional electronic portal imaging device (EPID). In our clinic, selected patients were treated under a phase I institutional review board-approved SBRT protocol for limited hepatic metastases from solid tumors. Before treatment planning multiple gold fiducial markers were implanted on the periphery of the tumor. During treatment the EPID was used in cine mode to collect the exit radiation and produce a sequence of images for each field. An in-house program was developed for calculating the location of the fiducials and their relative distance to the planned locations. Three case studies illustrate the utility of the technique. Patient A exhibited a systematic shift of 4 mm during one of the treatment beams. Patient B showed an inferior drift of the target of approximately 1 cm from the time of setup to the end of the fraction. Patient C had a poor setup on the first day of treatment that was quantified and accounted for on subsequent treatment days. Target localization throughout each treatment beam can be quickly assessed with the presented technique. Treatment monitoring with an EPID in cine mode is shown to be a clinically feasible and useful tool.

SU‐FF‐J‐12: A Phantom Study to Compare 2D Electronic Portal Imaging with 3D KV Cone‐Beam Imaging

Purpose: To compare patient positioning using a 2D Electronic Portal Imaging Device (EPID) with a 3D registration technique that uses a kV Cone Beam (CB) device. Introduction: The Elekta kV CB device represents a significant advancement in on‐line imaging for patient positioning and structure delineation. There exists an obvious interest in studying the robustness of this new technology. The phantom‐based study reported here compares the cone‐beam imaging capability with conventional EPID 2D imaging and registration. Additionally, a technique was developed to generate digitally reconstructed portal images (DRPIs) from the CBCT and compare them to EPID images. Materials and Methods: A Rando anthropomorphic phantom had surgical screws placed in the pelvic region. A treatment isocenter was selected in the pelvic region of the CT dataset of the phantom. The phantom was initially positioned using the CB volume imaging capability. Known shifts of the phantom were introduced and the phantom was repositioned using the CB. The CB corrections were compared to the corrections obtained using the internal fiducials and EPID imaging. Additionally, the EPID‐based positioning was mimicked by simulating the 2D imaging using the 3D CB data set. The digitally reconstructed images (DRR) were generated from the CBCT set and used as DRPIs. They were compared to both the diagnostic CT DRR's and the EPID images. Results: and Conclusion: The CB and EPID shifts were well correlated as determined by linear regression analysis. The correlation between the DRPIs and the EPIDs was even higher using the same analysis. However, on the shift‐by‐shift basis the CB was more accurate than the EPID: it gave more accurate shift determination than the EPID for approximately 90% test shifts. The DRPIs had the advantage over the CB 2Dimages because they included the treatment isocenter, field edges and distance scale.