Use Of Electronic Portal Imaging Devices Research Articles

In silico study of the electronic portal imaging devices sensitivity to parotid glands shrinkage during radiotherapy for nasopharyngeal cases

This computational investigation examines the capability of electronic portal imaging devices (EPIDs) in detecting anatomical changes during radiation therapy. A photon beam for 6 MV was simulated using compliant phase-space files via BEAMnrc (an EGSnrc user code). Initially, the dose calculation was carried out using DOSXYZnrc, an EGSnrc user code, on a water-based calibration phantom made of water. Phantoms, constructed utilizing patient computed tomography data, were subsequently used to simulate clinically relevant parotid gland (PG) shrinkage through image deformation using ImSimQA software. Two EPID geometries were implemented; a simple water slab, and a comprehensive multi-layer representation of the EPID (referred to as the 17-slab model). Using a 1%/1 mm gamma index the 17-slab geometry was sensitive to volume reductions in the PG ≥ − 26% from its original volume, while the water slab detected volume reduction of ≥ − 28.5%. In a clinical setting, replanning of head and neck patients is initiated when a reduction of 24–30% in PG volume is detected through cone-beam computed tomography. The sensitivity index, indicative of the signal to error ratio between the reference and evaluated images, showed an increase as the gland volume decreased for both models. Notably, the 17-slab model consistently more sensitive than the water slab. This computational study highlights the prospective use of EPIDs in monitoring and detecting clinically significant volume changes in the PG during radiation treatment. Advances in knowledge: While EPIDs are frequently employed for patient setup and alignment, this computational work constructed a virtual EPID to assess its detection abilities for the anatomical changes in the PG due to radiation exposure in head and neck cancer cases.

AAPM Task Group Report 307: Use of EPIDs for Patient-Specific IMRT and VMAT QA.

Electronic portal imaging devices (EPIDs) have been widely utilized for patient-specific quality assurance (PSQA) and their use for transit dosimetry applications is emerging. Yet there are no specific guidelines on the potential uses, limitations, and correct utilization of EPIDs for these purposes. The American Association of Physicists in Medicine (AAPM) Task Group 307 (TG-307) provides a comprehensive review of the physics, modeling, algorithms and clinical experience with EPID-based pre-treatment and transit dosimetry techniques. This review also includes the limitations and challenges in the clinical implementation of EPIDs, including recommendations for commissioning, calibration and validation, routine QA, tolerance levels for gamma analysis and risk-based analysis. Characteristics of the currently available EPID systems and EPID-based PSQA techniques are reviewed. The details of the physics, modeling, and algorithms for both pre-treatment and transit dosimetry methods are discussed, including clinical experience with different EPID dosimetry systems. Commissioning, calibration, and validation, tolerance levels and recommended tests, are reviewed, and analyzed. Risk-based analysis for EPID dosimetry is also addressed. Clinical experience, commissioning methods and tolerances for EPID-based PSQA system are described for pre-treatment and transit dosimetry applications. The sensitivity, specificity, and clinical results for EPID dosimetry techniques are presented as well as examples of patient-related and machine-related error detection by these dosimetry solutions. Limitations and challenges in clinical implementation of EPIDs for dosimetric purposes are discussed and acceptance and rejection criteria are outlined. Potential causes of and evaluations of pre-treatment and transit dosimetry failures are discussed. Guidelines and recommendations developed in this report are based on the extensive published data on EPID QA along with the clinical experience of the TG-307 members. TG-307 focused on the commercially available EPID-based dosimetric tools and provides guidance for medical physicists in the clinical implementation of EPID-based patient-specific pre-treatment and transit dosimetry QA solutions including intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) treatments.

Use of Amorphous Silicon (ASi) Electronic Portal Imaging Devices for Other applications for Linear Accelerator Quality Assurance

The success of radiation therapy depends critically on the accuracy of patient alignment in treatment position day after day. Electronic portal imaging devices (EPIDs) are widely used to monitor patient position during daily radiotherapy sessions. To verify other applications for medical linear accelerator quality assurance (linac QA), images of a 10×10 cm² and 18×18 cm² open field were acquired. The EPID was positioned at a fixed detector distance of 150 cm. The wedge profile and wedge factors with high level of accuracy were demonstrated. The profiles acquired using EPID deviated in shape and magnitude by up to 16% from the ion chamber profiles. The use of EPID for linac QA could be simplified by improving the available software analysis tools, thus making it more efficient. The study showed that the EPID aSi500 has the potential to be used as a relative dosimeter, making it a very simple and efficient tool for daily QA. All EPID measurements were performed using the linear accelerator model Varian DMX. The physical characteristics measured in this work suggest that the SLIC-EPID can be an efficient tool for daily QA.

15 Characterization of an aSi-1000 EPID response in integrated and continuous acquisition modes for SBRT dosimetry applications

Introduction The amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) mounted on linacs have been extensively studied and have shown dose-response characteristics suitable for dose verification. Their use for dosimetric purposes is already in clinical routine. However, the response of EPID in continuous acquisition mode was not so studied in literature, and in less extent when considering it for dosimetric applications in stereotactic body radiotherapy (SBRT) conditions (i.e. small fields, and high dose rates using flattening filter free (FFF) beams). The purpose of this work was therefore to characterize the EPID response in terms of relevant dosimetric properties, mainly for the potential use of continuous mode for further SBRT dosimetry with dynamic techniques. Methods The EPID used in this study was the aSi-1000 model installed on a Varian TrueBeamSTx. Its response was characterized in FFF photon beams with nominal energies of 6 and 10 MV and maximum available dose rates (1400 and 2400 MU/min, for 6 and 10 MV respectively). Different sets of images were acquired in both integrated and continuous modes, with EPID at 150 cm from the source, to study dose linearity, repeatability and reproducibility of response, field-size dependence, ghosting effect and response to energy spectrum variations. Before data acquisition, dark field and flood filed images were acquired for offset and gain corrections. As fluctuations in dose rate can potentially influence the response of EPID when using dynamic techniques with continuous acquisition mode, the linearity of EPID response was further evaluated by irradiating a 10 × 10 cm2 field in 20° and 90° arcs focusing on low MU range (3–100 MU), and was also compared to static irradiations. Results Conclusions The characterization of the aSi-1000 EPID response to relevant dosimetric properties was investigated, showing the feasibility of its use for dosimetry with FFF beams and high dose rates. Results for continuous mode, which differed from dosimetric non-linearity reported in previous published studies [ [1] McCurdy B.M.C. Greer P.B. Dosimetric properties of an amorphous-silicon EPID used in continuous acquisition mode for application to dynamic and arc IMRT. Med Phys. 2009; 36: 3028-3039 Crossref PubMed Scopus (73) Google Scholar ], also encourage the use of EPID for SBRT with dynamic techniques in future works for 3D dosimetry applications (e.g. quality assurance including in vivo dosimetry). The amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) mounted on linacs have been extensively studied and have shown dose-response characteristics suitable for dose verification. Their use for dosimetric purposes is already in clinical routine. However, the response of EPID in continuous acquisition mode was not so studied in literature, and in less extent when considering it for dosimetric applications in stereotactic body radiotherapy (SBRT) conditions (i.e. small fields, and high dose rates using flattening filter free (FFF) beams). The purpose of this work was therefore to characterize the EPID response in terms of relevant dosimetric properties, mainly for the potential use of continuous mode for further SBRT dosimetry with dynamic techniques. The EPID used in this study was the aSi-1000 model installed on a Varian TrueBeamSTx. Its response was characterized in FFF photon beams with nominal energies of 6 and 10 MV and maximum available dose rates (1400 and 2400 MU/min, for 6 and 10 MV respectively). Different sets of images were acquired in both integrated and continuous modes, with EPID at 150 cm from the source, to study dose linearity, repeatability and reproducibility of response, field-size dependence, ghosting effect and response to energy spectrum variations. Before data acquisition, dark field and flood filed images were acquired for offset and gain corrections. As fluctuations in dose rate can potentially influence the response of EPID when using dynamic techniques with continuous acquisition mode, the linearity of EPID response was further evaluated by irradiating a 10 × 10 cm2 field in 20° and 90° arcs focusing on low MU range (3–100 MU), and was also compared to static irradiations. The characterization of the aSi-1000 EPID response to relevant dosimetric properties was investigated, showing the feasibility of its use for dosimetry with FFF beams and high dose rates. Results for continuous mode, which differed from dosimetric non-linearity reported in previous published studies [ [1] McCurdy B.M.C. Greer P.B. Dosimetric properties of an amorphous-silicon EPID used in continuous acquisition mode for application to dynamic and arc IMRT. Med Phys. 2009; 36: 3028-3039 Crossref PubMed Scopus (73) Google Scholar ], also encourage the use of EPID for SBRT with dynamic techniques in future works for 3D dosimetry applications (e.g. quality assurance including in vivo dosimetry).

Evaluation of a fast method of EPID-based dosimetry for IMRT and Comparison with 3D EPID-based dosimetry system using conventional two- and three-dimensional detectors for VMAT

Introduction: Electronic portal imaging devices (EPIDs) could potentially be useful for intensity-modulated radiation therapy (IMRT) and VMAT QA. The data density, high resolution, large active area, and efficiency of the MV EPID make it an attractive option. However, EPIDs were designed as imaging devices, not dosimeters, and as a result they do not inherently measure dose in tissue equivalent media. EPIDose is a tool designed for the use of EPIDs in IMRT and VMAT QA that uses raw MV EPID images to estimate absolute dose planes normal to the beam axis in a homogeneous medi. However, because of the inherent challenges of the EPID-based dosimetry, validating and commissioning such a system must be done very carefully, by exploring the range of use cases and using well-proven “standards” for comparison. Converted EPID images were compared to 2D diode array absolute dose. To provide a 3D dosimetric evaluation of a commercial portal dosimetry system using 2D/3D detectors under ideal conditions using VMAT. Material & Method: A 2D ion chamber array, radiochromic film and gel dosimeter were utilized to provide a dosimetric evaluation of transit phantom and pre-treatment fluence’ EPID back-projected dose distributions for a standard IMRT & VMAT plan. In house 2D and 3D gamma methods compared pass statistics relative to each dosimeter and TPS dose distributions. Results: Fluence mode and transit EPID dose distributions back-projected onto phantom geometry produced 2D gamma pass rates in excess of 97% relative to other tested detectors and exported TPS dose planes when a 3%, 3mm global gamma criterion was applied. Use of a gel dosimeter within a glass vial allowed comparison of measured 3D dose distributions versus EPID 3D dose and TPS calculated distributions. 3D gamma comparisons between modalities at 3%, 3mm gave pass rates in excess of 92%. Use of fluence mode was indicative of transit results under ideal conditions with slightly reduced dose definition. Conclusions: 3D EPID back projected dose distributions were validated against detectors in both 2D and 3D. Cross validation of transit dose delivered to a patient is limited due to reasons of practicality and the tests presented are recommended as a guideline for 3D EPID dosimetry commissioning; allowing direct comparison between detector, TPS, fluence and transit modes. The results indicate achievable gamma scores for a complex VMAT plan in a homogenous phantom geometry and contributes to growing experience of 3D EPID dosimetry.

26. Evaluation of the EpiDream method for quality assurance (QA) of electron beams using a Varian electronic portal imaging device (EPID)

Introduction The aim of the study is to evaluate the use of EPID for electron beams QA defined by the 07/27/2007 French decision with a new approach in absorbed dose to water reconstruction: the EpiDream method. Methods The study was performed on two Varian LINACs, equipped with EPID (either with the AS500 or AS1000) for electron beams of 6, 9, 12, 15 and 18 MeV. Each EPID was calibrated with EpiDream application in order to obtain from the Grey Level (GL) image a dose matrix computed in water at the reference depth(zref) of the energy studied. For each energy, GL integrated images were corrected of dark field and acquired using AM Maintenance system(Varian) at the detector source distance (DSD) = 100 cm and DSD = (100 + zref) cm for different parameters : field sizes(6 × 6–20 × 20 cm2, delivered MU (3-300 MU), dose rate(100–300 MU/min). For each irradiation condition, a gamma index evaluation was performed between dose matrix computed by Eclipse TPS algorithm (eMC v13.6, Varian) and dose matrix computed by EpiDream method (DM EpiDream). For a 15 × 15 cm2 field size, a comparison was performed between radial and transverse dose profiles measured with a 0.125 cm3 ionization chamber(PTW31010) in water at zref for SSD = 100 cm and dose profiles obtained from DM EpiDream, considering: homogeneity and symmetry parameters, penumbra and correlation coefficient. Results Data analysis is in progress. However, first results are encouraging: At 9 MeV and for a DSD = 100 + zref, the mean gamma index value for 6 × 6, 10 × 10, and 15 × 15 cm2 field sizes is respectively: 99.9% (σ = 0.1%), 99.8% (σ = 0.32%), 96.3% (σ = 2.2%). Conclusions The EpidDream method would permit to perform electrons beams QA using EPID for all gantry angles by maximizing measurement reproducibility. The aim of the study is to evaluate the use of EPID for electron beams QA defined by the 07/27/2007 French decision with a new approach in absorbed dose to water reconstruction: the EpiDream method. The study was performed on two Varian LINACs, equipped with EPID (either with the AS500 or AS1000) for electron beams of 6, 9, 12, 15 and 18 MeV. Each EPID was calibrated with EpiDream application in order to obtain from the Grey Level (GL) image a dose matrix computed in water at the reference depth(zref) of the energy studied. For each energy, GL integrated images were corrected of dark field and acquired using AM Maintenance system(Varian) at the detector source distance (DSD) = 100 cm and DSD = (100 + zref) cm for different parameters : field sizes(6 × 6–20 × 20 cm2, delivered MU (3-300 MU), dose rate(100–300 MU/min). For each irradiation condition, a gamma index evaluation was performed between dose matrix computed by Eclipse TPS algorithm (eMC v13.6, Varian) and dose matrix computed by EpiDream method (DM EpiDream). For a 15 × 15 cm2 field size, a comparison was performed between radial and transverse dose profiles measured with a 0.125 cm3 ionization chamber(PTW31010) in water at zref for SSD = 100 cm and dose profiles obtained from DM EpiDream, considering: homogeneity and symmetry parameters, penumbra and correlation coefficient. Data analysis is in progress. However, first results are encouraging: At 9 MeV and for a DSD = 100 + zref, the mean gamma index value for 6 × 6, 10 × 10, and 15 × 15 cm2 field sizes is respectively: 99.9% (σ = 0.1%), 99.8% (σ = 0.32%), 96.3% (σ = 2.2%). The EpidDream method would permit to perform electrons beams QA using EPID for all gantry angles by maximizing measurement reproducibility.

TU‐D‐BRC‐00: The EPID Strikes Back

There has been increased deployment of gantry‐mounted linacs with onboard kilovoltage x‐ray imagers. Despite this trend, most clinics still employ beam's‐eye‐view (BEV) imaging using flat panel electronic portal imaging devices (EPIDs) as a highly useful means of verifying geometric localization pre‐ and post‐delivery. In recent years, there has been increased interest in the use of EPIDs for a variety of clinical and investigational applications ranging from megavoltage cone‐beam CT, in vivo dosimetry, patient‐specific QA to real‐time target localization for dynamic MLC‐based tumor tracking. In conjunction, there are several innovative EPID designs currently under investigation that hold great promise in bringing novel imaging and image‐guidance applications into clinical practice.This symposium is aimed at clinical and research‐oriented medical physicists who want to learn more about current and future megavoltage imaging technology and applications. In this symposium, we will examine the (i) interesting applications that make use of current EPID technology such as volumetric 4D image‐based models, real‐time position estimation for dynamic MLC tracking, etc., (ii) clinical applications such as in vivo EPID dosimetry and (iii) innovative high‐performance EPID designs and exciting future applications, including designs based on thick segmented scintillators, hybrid kV/MV imagers using segmented strip scintillators, and energy‐resolving EPIDs based on multilayer imagers and poly‐Silicon active matrix flat panel imagers.Learning Objectives – In this symposium, the attendees will learn Novel applications using currently available EPIDs Use of EPIDs to perform in vivo dosimetry Novel designs using high‐efficiency, energy resolving EPIDs and future applications of megavoltage imagingJ. Rottmann, Varian Medical SystemsP. Greer, Research Grant Funding from Radiation Oncology Institute (Grant ROI2013‐912), 2013–2014

SU-G-TeP2-01: Can EPID Based Measurement Replace Traditional Daily Output QA On Megavoltage Linac?

Purpose: To investigate the long term stability and viability of using EPID-based daily output QA via in-house and vendor driven protocol, to replace conventional QA tools and improve QA efficiency. Methods: Two Varian TrueBeam machines (TB1&TB2) equipped with electronic portal imaging devices (EPID) were employed in this study. Both machines were calibrated per TG-51 and used clinically since Oct 2014. Daily output measurement for 6/15 MV beams were obtained using SunNuclear DailyQA3 device as part of morning QA. In addition, in-house protocol was implemented for EPID output measurement (10×10 cm fields, 100 MU, 100cm SID, output defined over an ROI of 2×2 cm around central axis). Moreover, the Varian Machine Performance Check (MPC) was used on both machines to measure machine output. The EPID and DailyQA3 based measurements of the relative machine output were compared and cross-correlated with monthly machine output as measured by an A12 Exradin 0.65cc Ion Chamber (IC) serving as ground truth. The results were correlated using Pearson test. Results: The correlations among DailyQA3, in-house EPID and Varian MPC output measurements, with the IC for 6/15 MV were similar for TB1 (0.83–0.95) and TB2 (0.55–0.67). The machine output for the 6/15MV beams on both machines showed a similar trend, namely an increase over time as indicated by all measurements, requiring a machine recalibration after 6 months. This drift is due to a known issue with pressurized monitor chamber which tends to leak over time. MPC failed occasionally but passed when repeated. Conclusion: The results indicate that the use of EPID for daily output measurements has the potential to become a viable and efficient tool for daily routine LINAC QA, thus eliminating weather (T,P) and human setup variability and increasing efficiency of the QA process.

TU‐D‐BRC‐03: Future EPID Technology and Applications

There has been increased deployment of gantry‐mounted linacs with onboard kilovoltage x‐ray imagers. Despite this trend, most clinics still employ beam's‐eye‐view (BEV) imaging using flat panel electronic portal imaging devices (EPIDs) as a highly useful means of verifying geometric localization pre‐ and post‐delivery. In recent years, there has been increased interest in the use of EPIDs for a variety of clinical and investigational applications ranging from megavoltage cone‐beam CT, in vivo dosimetry, patient‐specific QA to real‐time target localization for dynamic MLC‐based tumor tracking. In conjunction, there are several innovative EPID designs currently under investigation that hold great promise in bringing novel imaging and image‐guidance applications into clinical practice.This symposium is aimed at clinical and research‐oriented medical physicists who want to learn more about current and future megavoltage imaging technology and applications. In this symposium, we will examine the (i) interesting applications that make use of current EPID technology such as volumetric 4D image‐based models, real‐time position estimation for dynamic MLC tracking, etc., (ii) clinical applications such as in vivo EPID dosimetry and (iii) innovative high‐performance EPID designs and exciting future applications, including designs based on thick segmented scintillators, hybrid kV/MV imagers using segmented strip scintillators, and energy‐resolving EPIDs based on multilayer imagers and poly‐Silicon active matrix flat panel imagers.Learning Objectives – In this symposium, the attendees will learn Novel applications using currently available EPIDs Use of EPIDs to perform in vivo dosimetry Novel designs using high‐efficiency, energy resolving EPIDs and future applications of megavoltage imagingJ. Rottmann, Varian Medical SystemsP. Greer, Research Grant Funding from Radiation Oncology Institute (Grant ROI2013‐912), 2013–2014

TU‐D‐BRC‐02: Free Information ‐ EPID‐Based In Vivo Dosimetry

There has been increased deployment of gantry‐mounted linacs with onboard kilovoltage x‐ray imagers. Despite this trend, most clinics still employ beam's‐eye‐view (BEV) imaging using flat panel electronic portal imaging devices (EPIDs) as a highly useful means of verifying geometric localization pre‐ and post‐delivery. In recent years, there has been increased interest in the use of EPIDs for a variety of clinical and investigational applications ranging from megavoltage cone‐beam CT, in vivo dosimetry, patient‐specific QA to real‐time target localization for dynamic MLC‐based tumor tracking. In conjunction, there are several innovative EPID designs currently under investigation that hold great promise in bringing novel imaging and image‐guidance applications into clinical practice.This symposium is aimed at clinical and research‐oriented medical physicists who want to learn more about current and future megavoltage imaging technology and applications. In this symposium, we will examine the (i) interesting applications that make use of current EPID technology such as volumetric 4D image‐based models, real‐time position estimation for dynamic MLC tracking, etc., (ii) clinical applications such as in vivo EPID dosimetry and (iii) innovative high‐performance EPID designs and exciting future applications, including designs based on thick segmented scintillators, hybrid kV/MV imagers using segmented strip scintillators, and energy‐resolving EPIDs based on multilayer imagers and poly‐Silicon active matrix flat panel imagers.Learning Objectives – In this symposium, the attendees will learn Novel applications using currently available EPIDs Use of EPIDs to perform in vivo dosimetry Novel designs using high‐efficiency, energy resolving EPIDs and future applications of megavoltage imagingJ. Rottmann, Varian Medical SystemsP. Greer, Research Grant Funding from Radiation Oncology Institute (Grant ROI2013‐912), 2013–2014

TU‐D‐BRC‐01: Novel Applications for Current EPID Technology

There has been increased deployment of gantry‐mounted linacs with onboard kilovoltage x‐ray imagers. Despite this trend, most clinics still employ beam's‐eye‐view (BEV) imaging using flat panel electronic portal imaging devices (EPIDs) as a highly useful means of verifying geometric localization pre‐ and post‐delivery. In recent years, there has been increased interest in the use of EPIDs for a variety of clinical and investigational applications ranging from megavoltage cone‐beam CT, in vivo dosimetry, patient‐specific QA to real‐time target localization for dynamic MLC‐based tumor tracking. In conjunction, there are several innovative EPID designs currently under investigation that hold great promise in bringing novel imaging and image‐guidance applications into clinical practice.This symposium is aimed at clinical and research‐oriented medical physicists who want to learn more about current and future megavoltage imaging technology and applications. In this symposium, we will examine the (i) interesting applications that make use of current EPID technology such as volumetric 4D image‐based models, real‐time position estimation for dynamic MLC tracking, etc., (ii) clinical applications such as in vivo EPID dosimetry and (iii) innovative high‐performance EPID designs and exciting future applications, including designs based on thick segmented scintillators, hybrid kV/MV imagers using segmented strip scintillators, and energy‐resolving EPIDs based on multilayer imagers and poly‐Silicon active matrix flat panel imagers.Learning Objectives – In this symposium, the attendees will learn Novel applications using currently available EPIDs Use of EPIDs to perform in vivo dosimetry Novel designs using high‐efficiency, energy resolving EPIDs and future applications of megavoltage imagingJ. Rottmann, Varian Medical SystemsP. Greer, Research Grant Funding from Radiation Oncology Institute (Grant ROI2013‐912), 2013–2014

Long-term two-dimensional pixel stability of EPIDs used for regular linear accelerator quality assurance

The long-term stability of three clinical electronic portal imaging devices (EPIDs) was studied to determine if longer times between calibrations can be justified. This would make alternatives to flood-field calibration of EPIDs clinically feasible, allowing for more effective use of EPIDs for dosimetry. Images were acquired monthly for each EPID as part of regular clinical quality assurance over a time period of approximately 3 years. The images were analysed to determine (1) the long-term stability of the EPID positioning system, (2) the dose response of the central pixels and (3) the long term stability of each pixel in the imager. The position of the EPID was found to be very repeatable with variations less than 0.3 pixels (0.27 mm) for all imagers (1 standard deviation). The central axis dose response was found to reliably track ion chamber measurements to better than 0.5%. The mean variation in pixel response (1 standard deviation), averaged over all pixels in the EPID, was found to be at most 0.6% for the three EPIDs studied over the entire period. More than 99% of pixels in each EPID showed less than 1% variation. Since the EPID response was found to be very stable over long periods of time, an annual calibration should be sufficient in most cases. More complex dosimetric calibrations should be clinically feasible.

Evaluation of a fast method of EPID‐based dosimetry for intensity‐modulated radiation therapy

Electronic portal imaging devices (EPIDs) could potentially be useful for intensity‐modulated radiation therapy (IMRT) QA. The data density, high resolution, large active area, and efficiency of the MV EPID make it an attractive option. However, EPIDs were designed as imaging devices, not dosimeters, and as a result they do not inherently measure dose in tissue equivalent media. EPIDose (Sun Nuclear, Melbourne, FL) is a tool designed for the use of EPIDs in IMRT QA that uses raw MV EPID images (no additional build‐up and independent of gantry angle, but with dark and flood field corrections applied) to estimate absolute dose planes normal to the beam axis in a homogeneous media (i.e. similar to conventional IMRT QA methods). However, because of the inherent challenges of the EPID‐based dosimetry, validating and commissioning such a system must be done very carefully, by exploring the range of use cases and using well‐proven “standards” for comparison. In this work, a multi‐institutional study was performed to verify accurate EPID image to dose plane conversion over a variety of conditions. Converted EPID images were compared to 2D diode array absolute dose measurements for 188 fields from 28 clinical IMRT treatment plans. These plans were generated using a number of commercially available treatment planning systems (TPS) covering various treatment sites including prostate, head and neck, brain, and lung. The data included three beam energies (6, 10, and 15 MV) and both step‐and‐shoot and dynamic MLC fields. Out of 26,207 points of comparison over 188 fields analyzed, the average overall field pass rate was 99.7% when 3 mm/3% DTA criteria were used (range 94.0–100 per field). The pass rates for more stringent criteria were 97.8% for 2 mm/2% DTA (range 82.0–100 per field), and 84.6% for 1 mm/1% DTA (range 54.7–100 per field). Individual patient‐specific sites as well, as different beam energies, followed similar trends to the overall pass rates.PACS number: 87.53.Dq; 87.66.Jj

SU-GG-T-215: Routine Linear Accelerator Electron Beam QA Using EPIDs

Purpose: To investigate the use of Electronic Portal Imaging Devices (EPIDs) for the routine quality assurance (QA) of linear accelerator (Linac) electron beams. Method and Materials: Routine electron beam QA is typically performed using radiographic film. The use of film for beam QA is time consuming and prone to many sources of error. EPIDs have become standard equipment on modern linacs, and their suitability for photon beam QA has been previously established. We have investigated the use of EPIDs for routing electron beam QA, including radiation field size, penumbra, flatness and symmetry. A Varian 2100iX with aSi imager was used. Service mode was used, as the linac does not support imaging with electron beams in clinical mode. Dark and flood field calibration of the EPID was obtained using the electron beams. EPID pixel response linearity with dose was verified for energies 6, 12, and 20 MeV with a 10×10 cm cone by exposing the imager to various MU. For radiation field size and beam flatness/symmetry QA, images were acquired using the 6×6 cm and 20×20 cm cones at energies of 6, 12 and 20 MeV. For comparison, Kodak XV films at a depth of 2 cm were exposed using the same configurations. The SDD for both film and EPID measurements was 105 cm. All films and images were analyzed using MATLAB code. Results: The EPID pixel response is linear with delivered dose. The radiation field size as measured by the EPID agreed with film to within 1.6 mm. Penumbra agreed to within 1.5 mm. It was not possible to obtain accurate flatness and symmetry results, but the EPID can be used for flatness and symmetry constancy. Conclusion: EPIDs can be used for the routine QA of electron beams. Radiation field size results are similar to those obtained with film.

Clinical Use of Electronic Portal Imaging

Accurate and routine target localization is necessary for successful outcome in radiation therapy treatments. Electronic portal imaging devices (EPIDs) provide an advanced tool with digital technology to improve target localization and maintain clinical efficiency. EPIDs are ubiquitous in the radiation therapy clinic, and they provide a powerful and flexible tool to collect and process data in a quantitative manner to improve treatment accuracy for virtually any treatment site. This manuscript provides an overview of the clinical implementation process for effective use of EPIDs. It continues with a review of correction strategies and finally highlights numerous examples of effective clinical application of EPID.

An evaluation of setup uncertainties for patients treated to pelvic sites

Purpose : Successful delivery of conformal fields requires stringent immobilization and treatment verification, as well as knowledge of the setup reproducibility. The purpose of this study was to compare the three-dimensional distribution of setup variations for patients treated to pelvic sites with electronic portal imaging devices (EPID) and portal film. Methods and Materials : Nine patients with genitourinary and gynecological cancers immobilized with custom casts and treated with a four-field whole-pelvis technique were imaged daily using an EPID and filmed once every five to seven treatments. The three-dimensional translational and rotational setup errors were determined using a technique that relies on anatomical landmarks identified on simulation and treatment images. The distributions of the translational and rotational variations in each dimension as well as the total displacement of the treatment isocenter from the simulation isocenter were determined. Results : Grouped analysis of all patients revealed average unidirectional translational deviations of less than 2 mm and a standard deviation of 5.3 mm. The average total undirected distance between the treatment and simulated isocenters was 8.3 mm with a standard deviation of 5 mm. Individual patient analysis revealed eight of none patients had statistically significant nonzero mean translational variations ( p < 0.05). Translational variations measured with film were an average of 1.4 mm less than those measured withEPID, but this difference was nto statistically significant. Conclusion : Translational variations measured in thsi study are in general agreement with previous studies. The use of the EPID in this study was less intrusive and may have resulted in less additional attention being given each imaging setup. This may explain the slightly larger average translational variations observed with EPID vs. film, and suggests that the use of EPIDs is a superior method for assessign the true extent of setup displacements. Although no statistically significant translational variations for the patient group overall were observed, 90% of patients had significant translational variations in at least one direction when analyzed separately. The margin to be added to the clinical target volume (CTV) to account for setup uncertainties will depend on whether it is possible to identify patients with significant translational variations, and to eliminate these displacements from routine treatments. The choice to eliminate these variations and to use a smaller CTV margin will have to be accompanied by stringent frequent position verification methods and repositioning.