Commissioning and Evaluation of an Electronic Portal Imaging Device-Based In-Vivo Dosimetry Software.

This study reports on our experience with the in-vivo dose verification software, EPIgray® (DOSIsoft, Cachan, France). After the initial commissioning process, clinical experiments on phantom treatments were evaluated to assess the level of accuracy of the electronic portal imaging device (EPID) based in-vivo dose verification. EPIgray was commissioned based on the company's instructions. This involved ion chamber measurements and portal imaging of solid water blocks of various thicknesses between 5 and 35 cm. Field sizes varied between 2 x 2 cm2 and 20 x 20 cm2. The determined conversion factors were adjusted through an additional iterative process using treatment planning system calculations. Subsequently, evaluation was performed using treatment plans of single and opposed beams, as well as intensity modulated radiotherapy (IMRT) plans, based on recommendations from the task group report TG-119 to test for dose reconstruction accuracy. All tests were performed using blocks of solid water slabs as a phantom. For single square fields, the dose at isocenter was reconstructed within 3% accuracy in EPIgray compared to the treatment planning system dose. Similarly, the relative deviation of the total dose was accurately reconstructed within 3% for all IMRT plans with points placed inside a high-dose region near the isocenter. Predictions became less accurate than < 5% when the evaluation point was outside the treatment target. Dose at points 5 cm or more away from the isocenter or within an avoidance structure was reconstructed less reliably. EPIgray formalism accuracy is adequate for an efficient error detection system with verifications performed in high-dose volumes. It provides immediate intra-fractional feedback on the delivery of treatment plans without affecting the treatment beam. Besides the EPID, no additional hardware is required. The software evaluates local point dose measurements to verify treatment plan delivery and patient positioning within 5% accuracy, depending on the placement of evaluation points

Novel dosimetry verification solutions for advanced radiation therapy

Improved optimisation of radiation dose delivery to tumours with improved sparing of normal tissues is an ongoing goal of radiotherapy practice. Advanced radiotherapy techniques are constantly improving to achieve this goal. However, these techniques are more complex to deliver. Hence verifying the source of dose errors is increasingly challenging. Accurate verification of treatment delivery for advanced radiotherapy becomes increasingly important in mitigating dose delivery errors which may compromise clinical outcome. This dissertation investigated treatment dosimetry verification for two different radiotherapy delivery systems i) Open gantry linear accelerator and ii) Helical TomoTherapy HI-ART ® (HT). Part (i) Open gantry linear accelerator - novel prototype hybrid EPID based dosimeters were developed for treatment verification to combine geometric and dosimetric verification in a single system. Initial work on dose response for standard EPIDs provided a more consistent understanding of EPID under-dose response for small monitor units (MUs). The dose response linearity of a standard a-Si EPID was evaluated for different combinations of linac, image acquisition settings and imaging data processing methods. EPID nonlinear response was demonstrated to be primarily due to gain ghosting affects in the a-Si photodiodes. This work has resolved some of the inconsistencies in the literature regarding EPID dose response and proposes a simple yet robust pixel-to-dose calibration method for EPID-based IMRT dosimetry..

An Efficient Approach to EPID Transit Dosimetry

Introduction: In order to maintain uniform standards in the accuracy of fractionated radiation therapy, quantification of the delivered dose per fraction accuracy is required. The pupose of this study was to investigate the feasibility of a transit dosimetry method using the electronic portal imaging device (EPID) for dose delivery error detection and prevention. Methods and Materials: In the proposed method, 2D predicted transit images were generated for comparison with online images acquired during treatment. Predicted transit images were generated by convolving through-air EPID measurements of each field with pixel-specific kernels selected from a library of pre-calculated Monte Carlo pencil kernels of various radiological thicknesses. The kernel used for each pixel was selected based on the calculated radiological thickness of the patient along the line joining the pixel to the virtual source. The accuracy of the technique was evaluated in homogeneous plastic water phantoms, a heterogeneous cylindrical phantom, and an anthropomorphic head phantom. Gamma analysis was used to quantify the accuracy of the technique for the various cases. Results: In the comparison between the measured and predicted images, an average of 99.4% of the points in passed a 3%/ 3 mm gamma for the homogeneous plastic water phantoms. Points for the heterogeneous cylindrical phantom analysis had a 94.6% passing rate. For the anthropomorphic head phantom, an average of 98.3% and 96.6% of points passed the 5%/3mm and 3%/3mm gamma criteria, respectively for all field sizes. Failures occurred typically at points when object thickness was changing rapidly or at boundaries between materials, and at the edges of large fields. Discussion: The results suggested that the proposed transit dosimetry method is a feasible approach to in vivo dose monitoring. The gamma analysis passing rates are within the accuracy needed for transit dosimetry. Future research efforts should include evaluation of the method for more complex treatment techniques and assessment of the sensitivity to changes in EPID or linac hardware, as well as characterization of any dependency the method may have on image ghosting or lag, gantry angle, or long-term stability

Transit dosimetry based on water equivalent path length measured with an amorphous silicon electronic portal imaging device

Abstract: Background and purpose: In vivo dosimetry is one of the quality assurance tools used in radiotherapy to monitor the dose delivered to the patient. The digital image format makes electronic portal imaging devices (EPIDs) good candidates for in vivo dosimetry. Currently there is no commercial transit dosimetry module, which could facilitate routine in vivo dosimetry with the EPID. Some centres are developing their in-house packages, and they are under assessment before introduction into routine clinical usage. The main purpose of this work was to develop the EPID as an in vivo dosimetry device. Materials and methods: Knowledge of a detector’s dose-response behaviour is a prerequisite for any clinical dosimetric application, hence in the first phase of the study, the dosimetric characteristics of eleven Varian a-Si500 EPIDs that are in clinical use in our centre were investigated. The devices have been in use for varying periods and interfaced with two different acquisition control software packages, IAS2 / IDU-II or IAS3 / IDU-20. Properties investigated include: linearity, reproducibility, signal uniformity, field size and dose-rate dependence, memory effects and image profiles as a function of dose. In the second phase, an EPID was calibrated using the quadratic method to yield values for the entrance and exit doses at the phantom or patient. EPID images for a set of solid water phantoms of varying thicknesses were acquired and the data fitted onto a quadratic equation, which relates the reduction in photon beam intensity to the attenuation coefficient and material thickness at a reference condition. The quadratic model was used to convert the measured grey scale value into water equivalent path length (EPL) at each pixel for any material imaged by the detector. For any other non-reference conditions, scatter, field size and MU variation effects on the image were corrected. The 2D EPL is linked to the percentage exit-dose for different thicknesses and field sizes, thereby converting the plane pixel values at each point into a 2D dose map at the exit surface of the imaged material. The off axis ratio is corrected using envelope and boundary profiles generated from the treatment planning system (TPS). The method was extended to include conformal and enhanced dynamic wedge (EDW) fields. A method was devised for the automatic calculation of areas (to establish the appropriate scatter correction) from the EPID image that facilitated the calculation of EPL for any field, and hence exit dose. For EDW fields, the fitting coefficients were modified by utilizing the Linac manufacturer’s golden segmented treatment tables (STT) methodology. Cross plane profiles and 2D dose distributions of EPID predicted doses were compared with those calculated with the Eclipse 8.6 treatment planning system (TPS) and those measured directly with a MapCHECK 2 device. Results: The image acquisition system influenced the dosimetric characteristics with the newer version (IAS3 with IDU-20) giving better data reproducibility and linearity fit than the older version (IAS2 with IDU-II). The irradiated field areas can be accurately determined from EPID images to within ± 1% uncertainty. The EPID predicted dose maps were compared with calculated doses from TPS at the exit. The gamma index at 3% dose difference (DD) and 3mm distance to agreement (DTA) resulted in an average of 97% acceptance for the square fields of 5, 10, 15 and 20 cm thickness solid water homogeneous phantoms. More than 90% of all points passed the gamma index acceptance criteria of 3% DD and 3mm DTA, for both conformal and EDW study cases. Comparison of the 2D EPID dose maps to those from TPS and MapCHECK shows that, more than 90% of all points passed the gamma index acceptance criteria of 3% dose difference and 3mm distance to agreement, for both conformal and EDW study cases. Conclusions: The quadratic calibration can effectively predict EPL and hence exit dose. Good agreement between the EPID predicted and TPS calculated dose distributions were obtained for open fields, conformal and EDW test cases. There were noteworthy deviations between EPID, TPS and MapCHECK doses on field edges. But it should be emphasised that, for practical in vivo dosimetry, these areas of reduced accuracy at the field edges are much less important. It is concluded that the EPID Quadratic Calibration Method (QCM) is an accurate and convenient method for online in vivo dosimetry and may therefore replace existing techniques

An Active Matrix Flat Panel Dosimeter (AMFPD) for inâ phantom dosimetric measurements

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135086/1/mp5012.pd

Implementation and evaluation of a transit dosimetry system for treatment verification

PURPOSE: To evaluate a formalism for transit dosimetry using a phantom study and prospectively evaluate the protocol on a patient population undergoing 3D conformal radiotherapy. METHODS: Amorphous silicon EPIDs were calibrated for dose and used to acquire images of delivered fields. The measured EPID dose map was back-projected using the planning CT images to calculate dose at pre-specified points within the patient using commercially available software, EPIgray (DOSIsoft, France). This software compared computed back-projected dose with treatment planning system dose. A series of tests were performed on solid water phantoms (linearity, field size effects, off-axis effects). 37 patients were enrolled in the prospective study. RESULTS: The EPID dose response was stable and linear with dose. For all tested field sizes the agreement was good between EPID-derived and treatment planning system dose in the central axis, with performance stability up to a measured depth of 18cm (agreement within -0.5% at 10cm depth on the central axis and within -1.4% at 2cm off-axis). 126 transit images were analysed of 37 3D-conformal patients. Patient results demonstrated the potential of EPIgray with 91% of all delivered fields achieved the initial set tolerance level of ΔD of 0±5-cGy or %ΔD of 0±5%. CONCLUSIONS: The in vivo dose verification method was simple to implement, with very few commissioning measurements needed. The system required no extra dose to the patient, and importantly was able to detect patient position errors that impacted on dose delivery in two of cases

On the development of a novel detector for simultaneous imaging and dosimetry in radiotherapy

Radiotherapy uses x-ray beams to deliver prescribed radiation doses that conform to target anatomy and minimise exposure of healthy tissue. Accuracy of dose delivery is essential, thus verification of dose distributions in vivo is desirable to monitor treatments and prevent errors from compromising patient outcomes. Electronic portal imaging devices (EPIDs) are commonly used x-ray imagers, however their non water-equivalent response complicates use for dosimetry. In this thesis, a Monte Carlo (MC) model of a standard EPID was developed and extended to novel water-equivalent configurations based on prototypes in which the high atomic number components were replaced with an array of plastic scintillator fibres. The model verified that full simulation of optical transport is not necessary to predict the standard EPID dose response, which can be accurately quantified from energy deposited in the phosphor screen. By incorporating computed tomography images into the model, its capacity to predict portal dose images of humanoid anatomy was also demonstrated. The prototype EPID’s water-equivalent dose response was characterised experimentally and with the MC model. Despite exhibiting lower spatial resolution and contrast-to-noise ratio relative to the standard EPID, its image quality was sufficient to discern gross anatomical structures of an anthropomorphic phantom. Opportunities to improve imaging performance while maintaining a water-equivalent dose response were identified using the model. Longer fibres increased efficiency and use of an extra-mural absorber maximised spatial resolution. Optical coupling between the scintillator fibres and the imaging panel may further improve performance. This thesis demonstrates the feasibility of developing a next-generation EPID for simultaneous imaging and dosimetry in radiotherapy. Such a detector could monitor treatment deliveries in vivo and thereby facilitate adaptations to treatment plans in order to improve patient outcomes

On the development of a novel detector for simultaneous imaging and dosimetry in radiotherapy

Radiotherapy uses x-ray beams to deliver prescribed radiation doses that conform to target anatomy and minimise exposure of healthy tissue. Accuracy of dose delivery is essential, thus verification of dose distributions in vivo is desirable to monitor treatments and prevent errors from compromising patient outcomes. Electronic portal imaging devices (EPIDs) are commonly used x-ray imagers, however their non water-equivalent response complicates use for dosimetry. In this thesis, a Monte Carlo (MC) model of a standard EPID was developed and extended to novel water-equivalent configurations based on prototypes in which the high atomic number components were replaced with an array of plastic scintillator fibres. The model verified that full simulation of optical transport is not necessary to predict the standard EPID dose response, which can be accurately quantified from energy deposited in the phosphor screen. By incorporating computed tomography images into the model, its capacity to predict portal dose images of humanoid anatomy was also demonstrated. The prototype EPID’s water-equivalent dose response was characterised experimentally and with the MC model. Despite exhibiting lower spatial resolution and contrast-to-noise ratio relative to the standard EPID, its image quality was sufficient to discern gross anatomical structures of an anthropomorphic phantom. Opportunities to improve imaging performance while maintaining a water-equivalent dose response were identified using the model. Longer fibres increased efficiency and use of an extra-mural absorber maximised spatial resolution. Optical coupling between the scintillator fibres and the imaging panel may further improve performance. This thesis demonstrates the feasibility of developing a next-generation EPID for simultaneous imaging and dosimetry in radiotherapy. Such a detector could monitor treatment deliveries in vivo and thereby facilitate adaptations to treatment plans in order to improve patient outcomes

Characterisation of a Novel Radiation Detector and Demonstration of a Novel Error Detection Algorithm for Application in Radiotherapy

Radiation detectors play an important role in radiology departments, particularly in relation to imaging and dosimetry. The significant advances achieved in material properties and high-quality electronic systems during previous decades has led to a continual expansion of their role and usage. In turn, this has had a concomitant impact upon the rapid progress of radiation detector technologies, specifically those utilised in medical imaging and dosimetry. This thesis aims to evaluate a radiation detector for a particular function, and to assess its suitability for said function within radiology and radiotherapy departments. Two novel radiation detectors, one for low energy imaging (kV) and another for radiotherapy (MV), are named Lassena (kV) and Lassena (MV) respectively. These detectors underwent an evaluation for the first time in order to assess their performance. Lassena (kV) was assessed in terms of image resolution and noise level to obtain the detective quantum efficiency (DQE) values representing image quality. DQE (0.5) values were 0.46-0.59 for three beam energies. Lassena (MV) was evaluated regarding its dosimetric properties, including linearity based on dose rate, reproducibility, and uniformity. Lassena (MV) has a high degree of short-term reproducibility, an acceptable pixel uniformity-response at high dose rates, and acceptable linearity with a coefficient of determination of 0.8624. Lassena (kV) displayed promising results whilst Lassena (MV) exhibited high sensitivity to radiation. A Monte Carlo system consisting of a linear accelerator and radiation detector was built and calibrated in order to assess dose verification applications within radiotherapy using a radiation detector. Anatomical changes during radiation therapy (such as parotid shrinkage and sinusitis for a nasopharyngeal case) were replicated. Analysis of computational EPID images started to warn of a risk of deviation from the planned dose at -26.3% volume loss of the parotid gland. This is most likely to happen in the third week of the treatment, however, the user must be aware of the limitations present due to anatomical overlapping and gamma analysis

2D transit dosimetry using electronic portal imaging device

The amorphous silicon electronic portal imaging device (a-Si EPID) was originally designed for positional verification in radiotherapy. Several feature of the a-Si EPID, such as the high-resolution detector array and ease of operation, have made this imaging device an attractive tool for dose measurements. The main challenge with a-Si EPID dosimetry is the deviation in scatter and dose response characteristics from a water-equivalent detector that makes the conversion of EPID signal to dose not straightforward. The aim of this thesis is to develop a model to perform 2D transit dosimetry for patient-specific treatment verification with a-Si EPID. The transit model can be used for both pre-treatment and actual treatment verification to ensure safety in different stages of the radiotherapy process. The model was based on a quadratic equation that relates the reduction in radiation intensity, represented by the ratio of exit to entrance dose, to the water-equivalent path length (EPL) of the attenuator. Coefficients in the quadratic equation were derived from a set of calibration dose planes measured for a reference beam with water phantoms of known thicknesses. Two sets of coefficients were derived separately from calibration dose planes measured with EPID and ionisation chamber (IC) in water. Consequently, with two sets of coefficients, the EPL of any attenuator can be calculated using either EPID measured dose planes or treatment planning system (TPS) computed dose planes for the treatment field to be verified. The calculated EPL, which is a property of the attenuator and independent of the dosimeter, was used to link the different dosimetry systems and provide a two-way relationship for either: (Path 1) reconstruction of in-phantom or in-vivo dose from EPID measured dose planes, for comparison with TPS planned dose; or (Path 2) prediction of EPID transit dose from TPS computed dose planes, for comparison with EPID measurement during treatment. The developed model was first tested with homogeneous and heterogeneous slab phantoms using open, enhanced dynamic wedge (EDW) and intensity modulated radiation therapy (IMRT) fields. Results showed that the model could accurately detect deviation between delivered and planned doses. Further evaluation with an anthropomorphic pelvic phantom and 65 test fields (open, 3D conformal, EDW, IMRT) at different gantry angles showed a mean gamma pass rate (4%/4mm criterion) of 97.6% (range: 90.0% to 100%) for in-phantom exit dose comparisons (Path 1) and 97.1% (range: 92.9% to 99.8%) for EPID transit dose comparisons (Path 2). In addition, the methods in Path 1 were expanded to reconstruct dose at other levels besides the exit level. In-phantom isocentre dose comparisons resulted in a mean gamma pass rate of 98.2% (range: 91.7% to 100%). Finally, clinical feasibility of the EPID transit dosimetry model was demonstrated for three patients (11 3D conformal fields, 18 verifications) who were undergoing radiotherapy treatment at the pelvic region. Gamma analyses with 5%/5mm criterion resulted in a mean pass rate of 97.0% (range: 92.4% to 99.6%) and 98.6% (range: 96.1% to 100%) for in-vivo comparisons at the exit level and isocentre level respectively (Path 1). The mean gamma pass rate for EPID transit dose comparisons (Path 2) was 95.6% (range: 90.7% to 99.9%). In conclusion, the 2D EPID transit dosimetry model developed in this thesis has been proven to be valid and suitable for clinical implementation. The model is: (1) practical, involving only general measurements and does not require any modification to the EPID panel, (2) generic, with the methods applicable to all a-Si EPID and TPS regardless of manufacturer and (3) flexible, allowing users to verify the accuracy of treatment delivery either at multiple planes in-vivo or at the EPID level. These are important characteristics to encourage widespread implementation of EPID transit dosimetry in different clinical setting for safer radiotherapy