Browsing by Author "Wang, Chu"
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Item Open Access Dose Verification and Monte Carlo Modeling of an Image-Guided Small Animal Radiotherapy Irradiator & Investigation of Occupational Radiation Exposure to Interventional Radiologists from Use of Fluoroscopic Imaging(2024) Dominici, Jessica DProject 1 (Chapter 2): Dose Verification andMonte Carlo Modeling of an Image-GuidedSmall Animal Radiotherapy Irradiator
Purpose: Preclinical trials play a crucial role in advancing the understanding of cancer biology and developing effective therapeutic interventions. The purpose of this project is to simulate and validate beam output of a Small Animal Radiotherapy Research Platform (SARRP, xStrahl) with both physical dosimetry and Monte Carlo simulation models.
Materials and Methods: The SARRP console was set up to deliver an intended dose of 8Gy (4 Gy anterior-posteriorly (AP), 4 Gy posterior-anteriorly (PA)) in 142 seconds to a flat mouse phantom. The x-ray irradiation parameters were set to 13 mA, 220 kVp, with a 33.725 cm source to surface distance. Beam filtration included 0.8 mm Be (inherent) and 0.15 mm Cu (added), with collimation set to 40x30 mm. Dose verification was conducted through two methods: utilizing an energy-calibrated MOSFET dosimeter and employing Monte Carlo Simulations using Monte Carlo N- Particle Transport (MCNP). MOSFET Calibration encompassed four setups to ensure precision. The first two involved calibrating the MOSFET with an ion chamber in air at 0 degrees (Setup 1) and 180 degrees (Setup 2). The subsequent two setups calibrated the MOSFET positioned inside the phantom (Setups 3 and 4) with an ion chamber in air. After the calibrations, the MOSFET, placed inside the phantom, received the intended 4 Gy dose for verification. The MCNP simulation comprised two stages: a point source simulation and a simulation of the x-ray tube. For the point source, the SARRP geometry was replicated, with the x-ray tube modeled as a collimated point source. The x-ray tube simulation entailed modeling components of the xray tube. Validation methods included comparing energy spectra, Half Value Layer (HVL) testing, and film analysis of the anode heel effect.
Results: In the dose verification, Setup 1 exceeds the intended 4 Gy dose by 7.72%, while Setup 2 underdoses by 2.96%, resulting in a cumulative overdose of 2.38% for Setups 1 and 2. Setup 3 aligns with the intended 4 Gy dose, underdosing slightly by 0.14%, while Setup 4 underdoses by 2.31%. The cumulative dose for Setups 3 and 4 totals 7.90 Gy, indicating a 1.27% underdose. The two calibration techniques demonstrate a difference of 3.6%. Calibration in air is the preferred method due to the ionization chamber also being present in air. Point Source Simulation yielded doses of (4.27 ± 0.02) Gy (AP) and(3.77 ± 0.02) Gy (PA). X-ray Tube Simulation resulted in (3.95 ± 0.02) Gy (AP). Energy spectrum of the MCNP model showed good agreement with the manufacturer model in key spectral characteristics (peaks, mean energies). HVL comparison showed good agreement with only a 0.5% difference between simulated and experimental half value thicknesses. The anode-heel effect analysis was inconclusive.
Conclusions: The dose verification processes establish the SARRP’s efficacy in delivering the intended radiation dose. The integration of advanced measurement techniques set a benchmark for small animal dosimetry and ultimately strengthens the reliability of radiation doses in preclinical studies.
Project 2 (Chapter 3): Investigation of Occupational Radiation Exposure to InterventionalRadiologists from Use of Fluoroscopic Imaging
Purpose: The purpose of this project is to investigate radiation exposure among Interventional Radiology (IR) physicians using fluoroscopic imaging through experimental data collection and retrospective analysis, with objectives to understand Automatic Exposure Rate Control mechanisms, assess exposure rates to operators, and identify trends amongIR physicians.
Materials and Methods: Three interventional fluoroscopes were investigated: Philips AlluraClarity Xper FD 20/15, Philips Allura Xper FD20, and GE Discovery IGS 740. Two phantoms were employed to replicate patient and operator. The “patient” phantom was comprised of water-equivalent slabs (5cm thick, 30cm x 30cm). The “operator” phantom (Atom Dosimetry Labs Adult Male phantom) was placed beside the table and covered with a 0.25 mm lead apron. A Ludlum 9DP pressurized ion chamber was positioned at collar level of the operator phantom. Parameters varied included patient thickness (20cm-40cm), collimation, and fluoroscopy and acquisition modes. Exposure (mR) to “operator”were measured and normalized to number of x-ray pulses. Retrospective analysis used Radiation Dose Structured Report data for an 8-month period. Physician caseload, averageCumulative Air Kerma (CAK) by physician, and total CAK by physician were determined.
Results: Based on measured data, acquisition mode exhibits longer pulse widths (7.20x-31.3x) and higher tube current (1.48x-7.58x) compared to fluoroscopy for all units. Tube voltage increases with phantom thickness in both fluoroscopy and acquisition mode for all units. Increasing phantom size and collimated field size elevate the operator exposure rate for all C-arms. Larger field sizes contribute to higher exposure rates compared to small field sizes (4.91x-6.27x fluoroscopy, 5.51x-8.23x acquisition). Additionally, acquisition mode contributes to higher exposure rates (23.4x–107.1x) than fluoroscopy. Analysis of proceduraldata identified trends in case distribution and dose to patients across physicians.
Conclusions: Overall, patient size, collimation settings, and fluoroscopy vs. acquisition mode were identified as significant contributors to operator exposure rates. Outliers among IR physicians highlighted the need for targeted interventions to mitigate excessive radiation exposure.
Item Open Access Evaluation of Patient Effective Dose of Neurovascular Imaging Protocols of a C-arm Cone-beam CT & Estimation of Current Source Radioactivity of a Cs-137 Irradiator(2012) Wang, ChuPurpose:
(Project 1) The purpose of this study was three-fold: 1) to estimate the organ doses and effective dose (ED) for patients undergoing neurovascular imaging protocols, 2) to study the effect of beam collimation on ED for 3-D imaging protocols, and 3) to derive protocol-specific DAP-to-ED conversion factors.
(Project 2) The Cs-137 irradiator is one of the most commonly used irradiation device in radiobiological research. The purpose of this study is to develop a simple method to estimate the current source radioactivity of a Cs-137 irradiator (Mark I-68A, JL Shepherd).
Material and Methods:
(Project 1) A cone-beam CT system (Philips Allura Xper FD20/20) was used to measure the organ doses for seven 3-D (cone-beam CT and 3-D Rotational Angiography protocols) and eight 2-D (fluoroscopy and digital subtraction angiography) imaging protocols. Organ dose measurements were performed on an adult male anthropomorphic phantom (CIRS, Norfolk, VA) with 20 MOSFET detectors (Best Medical Canada, Ottawa, Canada) placed in selected organs. The dose area product (DAP) values were recorded from console. The ED values were computed by multiplying measured organ doses to corresponding ICRP 103 tissue weighting factors. The ED of four 3-D imaging protocols were also measured with standardized beam collimation to compare with the ED associated with the same protocols without beam collimation.
(Project 2) Three positions along the peak-dose irradiation direction within the irradiation chamber were picked as the reference dosimetry positions. Individual dose rate at each of these positions was measured by an ion chamber in "Gy/sec", as well as estimated by Monte Carlo simulation in "Gy/primary event". The source activity, "disintegration/sec", was then derived from these two sets of values and corrected by the branching ratio of the main 662 keV emission.
Results:
(Project 1) For the seven 3-D imaging protocols with uncollimated setting, the EDs ranged from 0.16 mSv to 1.6 mSv, and the DAP-to-ED conversion factors range from 0.037 to 0.17 mSv/Gy∙cm2. For four protocols with beam collimation, ED was reduced approximately by a factor of 2, and the DAP-to-ED conversion factors by approximately 30%. For the eight 2-D imaging protocols, the ED rates ranged from 0.02 mSv/sec to 0.04 mSv/sec (for DSA) and from 0.0011 mSv/sec to 0.0027 mSv/sec (for fluoroscopy), and the DAP-to-ED conversion factors range from 0.045 to 0.068 mSv/Gy∙cm2 (for DSA) and factors range from 0.0029 to 0.059 mSv/Gy∙cm2 (for fluoroscopy).
(Project 2) For the irradiator in question, the source activity, as of Nov. 17, 2011, was estimated to be 2770 Curies. The current activity from the manufacturer was calculated to be 5900 Curies.
Conclusion:
(Project 1) We have measured ED for standard adult neuro imaging protocols in a C-arm cone-beam CT system. Our results provide a simple means of ED estimation using DAP values from console in the C-arm cone-beam CT system.
(Project 2) Our method offers a convenient means to estimate the source activity. The result was compared to the value computed from the manufacturer. We have found discrepancies between the two: 41%, 86%, and 97%, assessed at location 1, 2, and 3, respectively.
Item Open Access Improvements in Small Animal Dosimetry: CIX3 Irradiator Characterization, Novel Phantom Investigation, and Shepherd Cs-137 Irradiator Dose Uniformity Analysis(2023) Filip, Kevin TProject 1 (Chapter 2): X-Strahl CIX3 CharacterizationPurpose: In Fall 2021 Duke University purchased an Xstrahl CIX3 Cabinet Xray Irradiator. A characterization of this machine was performed to determine its dosimetry characteristics and best practices for radiobiological studies at Duke. Basic irradiator acceptance tests were expanded on to fully characterize this new machine. Two unique aspects of this machine were of particular interest. First a unique (non-uniform) filter design was investigated to determine if it has unintended side effects on field conformity. Second the lack of accessible cable ports made thermoluminescent dosimeters (TLDs) the primary dosimeters for experiments. For this reason additional investigations were conducted characterizing TLDs. Materials and Methods: To assess the dosimetric properties of this new machine the following parameters were investigated: output consistency, beam quality, field uniformity, and exposure rates. Output consistency was measured by comparing expected and observed max energy in kVp using a Piranha X-ray multimeter. Beam quality was measured as half value layer of aluminum and compared to expected results from Spekcalc. Spekcalc was used to determine energy fluence and mean energy of the spectrum. Field uniformity was assessed using Gafchromic ™ EBT3 film on an Epson Expression 10000 XL scanner with lateral response artifact correction factors and film calibrated using a NIST traceable 0.18 cc ion chamber. Exposure rates were characterized using a NIST traceable 0.18cc ion chamber and varying filtration, tube energy (kVp), and tray position for all available configurations. TLD energy response, positional response, and batch correction factor techniques were characterized on this machine. Energy response was determined by irradiating TLD’s to a range of energies (70-300 kVp) and the charge response reliv ative to the exposure (50 R) received was determined. Positional response in the field was investigated using the Duke Radiation Dosimetry Laboratory (DRDL) TLD holder and Gafchromic ™ EBT3 film. The relative exposure each TLD received was determined and compared to the ion chamber exposure. Results: The CIX3 had consistent energy output as measured by max energy conformance. The average difference from input voltage to output voltage was 0.84 % with the worst being 1.6 % (150 kVp, 1 mm copper Filtration). Theoretical estimations (Spekcalc) of the beam quality had good agreement with measured half value layers (using Piranha) with an average difference of 1.36 % and worst error of 2.99 % across the energy ranges sampled (50-150 kVp). Field uniformity results indicated general conformance to machine data (90 % within 25.9 cm diameter field) but some non-uniformities were identified. Areas of higher dose (105-110%) relative to the center were observed in the upper right quadrant of the field (from beam eye perspective). TLD energy response followed expected over-response in lower energy ranges, reducing to a normal response as energy increases. The highest over response was at 70 kVp, a 15 % over response compared to 300 kVp (the max energy of the CIX3). The film study to determine TLD positional response determined there were unequal exposures to the 50 TLDs. The trend observed an increase in exposure consistent with the field uniformity results in which the dose relative to the center of the field increased by 5-10 % towards the upper right quadrant. This effect was more pronounced at the lower energy level sampled (90 kVp). Conclusions: The full characterization of the CIX3 was very important to understand the nuances of the new machine. Conformance in beam quality results gave good indications that the machine is operating as designed and radiobiological studies expect to have consistent results between experiments under like conditions. The unique field uniformity results observed could help inform future experimental planning. More importantly it is an extremely important finding for TLD dose calibration. Since TLDs experience at most 6-7 % over exposure compared to the ion chamber it is important to use these findings in future calibrations. Project 2 (Chapter 3): Dose Depth of Small AnimalWater Phantom Purpose: Much is understood about the midpoint dose estimation of small animal phantoms and it has been the focus of DRDL to conduct dosimetry using this value. However, little data was available on the dose depth profile of small animal phantoms. This investigation sought to fill in that dose depth data, compare doses under varying experimental conditions in order to fully understand how the dose is distributed along the beam axis for small animal phantoms. Materials and Methods: A small animal water phantom (50 cc water vial) was characterized on the CIX3 using Gafchromic ™ EBT3 film. Film was calibrated using a NIST traceable 0.18 cc ion chamber. The phantom was irradiated with strips of film (15 total) varying filtration and whether a backscatter plate (uniform piece of acrylic) was included. The film was scanned and analyzed using Film QA software and aggregated dose depth profiles were determined using R Studio and Excel. Results: The dose depth of the phantom was characterized with a coefficient of variation of about 2-3 % across all depths and configurations. The inclusion of the backscatter plate improved the dose uniformity by an average of 2.14 % with most improvements coming from the bottom half of the phantom (closest to the backscatter plate). Average dose rates under each configuration were determined. The midpoint dose rate was found to have good conformance (within 0.5%) to the averaged dose rate across the depth. The dose rate increased by 33 % when using the backscatter plate due to the increased backscatter spectrum and the inverse square law effects. Conclusions: This data gave increased certainty in using the midpoint dose as a surrogate measure for whole body dose averages in small animal phantoms. The improvement in dose uniformity when using the backscatter plate seems like a promising addition to future experimental configurations. However in application the size of the backscatter plate makes it unusable for large experiment samples and it should be implemented only in specific studies as determined during the dosimetry consult with DRDL. Project 3 (Chapter 4): Novel Mouse Phantom Investigation Purpose: In recent years DRDL developed a novel mouse phantom based on feedback from researchers. This novel phantom flattened the mouse to mimic a mouse laying on an irradiation platform, especially when sedated. This investigation sought to determine if the novel ’flat’ phantom made of polymethyl methacrylate (acrylic) (PMMA) had a significant difference in dosimetry when compared with a standard cylindrical phantom made of soft tissue equivalent material. Materials and Methods: To compare the dosimetric differences each phantom a FLUKA Monte Carlo simulation was compared with experimental results from TLDs on the CIX3. In the simulation the dose was compared using a midpoint dose volume. In the experimental design TLDs were used to determine the dose at the midpoint of each phantom. The dose rates were analyzed and compared to determine if there was a significant difference. Results: The Monte Carlo results indicated there were very slight differences between phantom rates. The cylinder phantom had a dose rate of 1.83±0.038 Gy/min while the flat phantom had a midpoint dose of 1.81 ± 0.045 Gy/min, 1.1 percent lower than the cylindrical phantom. An unpaired t-test was performed to determine if the samples were different and was found to give a p-value of 0.71 which gives a high probability that the sampled data are not significantly different. The experimental results were found to be similar. The cylinder phantom had a dose rate of 1.77 ± 0.05 Gy/min while the flat phantom had a midpoint dose of 1.81 ± 0.045 Gy/min, 2.25 percent higher than the cylindrical phantom. Again a t-test was performed and these were determined to not be significantly different (p = 0.31). Conclusions: The flat phantom therefore is very similar to the dosimetry found in the cylinder phantom. Small variations due to material properties, height of phantom, scatter material, and attenuating material all balanced out to provide dosimetry properties that are similar. This means that the cheaper to manufacture flat phantom is just as good as the much more expensive cylinder phantom and both can be used for small animal dosimetry. Project 4 (Chapter 5): Shepherd Mk I - 68A Dose Uniformity Purpose: In 2009 a study was conducted to determine the dose uniformity in a Shepherd Mk I-68A Cs-137 irradiator. Since then vast improvements have been made on film design and software to analyze scanned film with improved accuracy. A follow up study was designed to revisit this previous characterization and update the dose uniformity of the irradiator cavity using these new dosimeters and techniques. Materials and Methods: Gafchromic ™ EBT3 film was calibrated using a NIST traceable 0.18 cc ion chamber on the Shepherd irradiator in each of the three positions available. Large film sheets were then irradiated in a 2 mm acrylic holder in all positions under rotating and non-rotating configurations. The scanned film was analyzed using FilmQA software, R studio, and Excel to determine the dose uniformity relative to the dose in the center. Results: An ion chamber sample was compared to the film results and found to be in good agreement (within 1 %) which indicated the film was appropriately irradiated, scanned, and calibrated. Rotating dose distributions in positions 2 and 3 were nearly equivalent to manufacturer predicted isodose distributions with noted discrepancies at the edges of the field. At a height of 15 cm to achieve dose uniformity of 100 % ± 5 % the rotating tray has a usable radius of 7 cm from the center in position 2 and 8 cm in position 3. Stationary dose distributions were compared to previous uniformity and found to be in general agreement in the center. However the isodose mapping previously characterized did not include a scanner lateral response artifact correction factor which indicated a better uniformity than what was found in this experiment. Position 1 results were similar to previous dose distributions and most importantly confirmed the positioning of the source in the chamber. Conclusions: The updated dose uniformity data provides QA feedback as part of a larger dosimetry program for Duke University. These results indicated that this irradiator was and still is performing as expected and no mechanical failures have caused a source to become misaligned or any major changes to the expected dosimetry.
Item Open Access Investigation of Occupational Dose to Interventional Radiologists(2023) Tysinger, Millicent PAbstractProject 1: Measuring the Effects on Operator Dose of Changing Clinical Settings Purpose: This study was initiated as part of a multi-faceted investigation of occupational dose to Interventional Radiologists consequential to their role as operators of fluoroscopy equipment. This project aims to qualitatively evaluate general dose reduction techniques, including clinical protocol settings on different interventional fluoroscopes to determine the specific impact on operator dose at Duke University Hospital. Materials and Methods: For each unit, analogous baseline settings were selected with a general abdominal protocol. The patient table was set to a source-to-object distance (SOD) of 62.23 cm (24.5 in) and a patient phantom was placed in the beam as a scatter medium similar to a typical patient abdomen. An anthropomorphic “operator” phantom was draped with a lead apron and positioned to one side of the patient table with an ion chamber placed at collar level. The ion chamber was placed such that the center of the active volume was 38.1 cm (15 in) lateral to and 63.5 cm (25 in) inferior from the center of the flat-paneled detector. A series of scans was taken on each unit, with each one having a selected variable changed, and the exposure readings from the ion chamber were recorded for comparison. Results: The effects on operator exposure rate of personnel height, contour shield use, cine mode, magnification, low dose mode, and source-to-image distance (SID) were analyzed. Operator height was found to have a larger effect on exposure rate reduction with distance than anticipated. Use of the contour shield reduced the operator exposure rate by over 90% on each unit. Use of cine mode drastically increased the exposure rate to the operator, while magnification, low dose mode, and decreasing SID all resulted in lower exposure rates. Conclusions: Operators can utilize these results to contextualize the effects of their own dose reduction techniques. Knowledge and familiarity of the techniques which offer the best exposure rate reduction can guide radiation protection practices among staff and help to optimize occupational doses. Project 2: Developing a Conceptual Framework for Analyzing the Radiation Dose Structured Report Purpose: When investigating occupational dose to Interventional Radiologists, it is important to be able to accurately compare metrics related to dose from historical procedures. The Radiation Dose Structured Report (RDSR) provides characteristic data from historical procedures. With an appropriate framework for analyzing RDSR data, performance metrics between operators or units can be compared, and identified trends can be used to develop dose reduction techniques specific to the organization. Materials and Methods: RDSR data from five interventional fluoroscopy systems (K1 – K5) was extracted for a three-year period from July 2019 through August 2022, and multiple metrics of comparison were selected for analysis. To determine differences in machine output, air kerma rates of similar procedures were compared, as well as the overall machine utilization for each year. Differences in operator-selectable variable were compared through air kerma rate per procedure, fluoroscopy time per procedure (limited to central line procedures), and operator caseload makeup. Results: Machine comparison of air kerma rates showed a consistently higher median and variability on the Philips Allura systems compared to the other three units. The Philips AlluraClarity unit in suite K2 was noticeably under-utilized by Interventional Radiology staff due to it being the primary fluoroscope used by Neurosurgery staff who were outside the scope of this investigation. Operator air kerma rates were compared from August 2021 through August 2022 and largely showed similar median values and variability. Fluoroscopy time per procedure fit to lognormal distributions and compared through their distribution parameter μ showed a median value which dipped during the second year for most providers. One operater also had a consistently higher median time per procedure for all three years. Conclusions: The analysis described by this framework provides a means of utilizing RDSR data to compare performance of interventional procedures. Continual local analysis of these metrics can be used to guide operator training to ensure that occupational doses are optimized to be as low as reasonably achievable. This is an initial approach that can be expanded through investigation and further characterization of procedure data included in the RDSR.
Item Open Access Radiation Dose Estimation for Pediatric Patients Undergoing Cardiac Catheterization(2015) Wang, ChuPatients undergoing cardiac catheterization are potentially at risk of radiation-induced health effects from the interventional fluoroscopic X-ray imaging used throughout the clinical procedure. The amount of radiation exposure is highly dependent on the complexity of the procedure and the level of optimization in imaging parameters applied by the clinician. For cardiac catheterization, patient radiation dosimetry, for key organs as well as whole-body effective, is challenging due to the lack of fixed imaging protocols, unlike other common X-ray based imaging modalities.
Pediatric patients are at a greater risk compared to adults due to their greater cellular radio-sensitivities as well as longer remaining life-expectancy following the radiation exposure. In terms of radiation dosimetry, they are often more challenging due to greater variation in body size, which often triggers a wider range of imaging parameters in modern imaging systems with automatic dose rate modulation.
The overall objective of this dissertation was to develop a comprehensive method of radiation dose estimation for pediatric patients undergoing cardiac catheterization. In this dissertation, the research is divided into two main parts: the Physics Component and the Clinical Component. A proof-of-principle study focused on two patient age groups (Newborn and Five-year-old), one popular biplane imaging system, and the clinical practice of two pediatric cardiologists at one large academic medical center.
The Physics Component includes experiments relevant to the physical measurement of patient organ dose using high-sensitivity MOSFET dosimeters placed in anthropomorphic pediatric phantoms.
First, the three-dimensional angular dependence of MOSFET detectors in scatter medium under fluoroscopic irradiation was characterized. A custom-made spherical scatter phantom was used to measure response variations in three-dimensional angular orientations. The results were to be used as angular dependence correction factors for the MOSFET organ dose measurements in the following studies. Minor angular dependence (< ±20% at all angles tested, < ±10% at clinically relevant angles in cardiac catheterization) was observed.
Second, the cardiac dose for common fluoroscopic imaging techniques for pediatric patients in the two age groups was measured. Imaging technique settings with variations of individual key imaging parameters were tested to observe the quantitative effect of imaging optimization or lack thereof. Along with each measurement, the two standard system output indices, the Air Kerma (AK) and Dose-Area Product (DAP), were also recorded and compared to the measured cardiac and skin doses – the lack of correlation between the indices and the organ doses shed light to the substantial limitation of the indices in representing patient radiation dose, at least within the scope of this dissertation.
Third, the effective dose (ED) for Posterior-Anterior and Lateral fluoroscopic imaging techniques for pediatric patients in the two age groups was determined. In addition, the dosimetric effect of removing the anti-scatter grid was studied, for which a factor-of-two ED rate reduction was observed for the imaging techniques.
The Clinical Component involved analytical research to develop a validated retrospective cardiac dose reconstruction formulation and to propose the new Optimization Index which evaluates the level of optimization of the clinician’s imaging usage during a procedure; and small sample group of actual procedures were used to demonstrate applicability of these formulations.
In its entirety, the research represents a first-of-its-kind comprehensive approach in radiation dosimetry for pediatric cardiac catheterization; and separately, it is also modular enough that each individual section can serve as study templates for small-scale dosimetric studies of similar purposes. The data collected and algorithmic formulations developed can be of use in areas of personalized patient dosimetry, clinician training, image quality studies and radiation-associated health effect research.