Doctoral Degrees (Medical Physics)
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Browsing Doctoral Degrees (Medical Physics) by Author "Du Plessis, F. C. P."
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Item Open Access Accuracy of patient-specific dosimetry using hybrid planar-SPECT/CT imaging: a Monte Carlo study(University of the Free State, 2021-07) Morphis, Michaella; Van Staden, J. A.; Du Raan, H.; Du Plessis, F. C. P.Introduction: Theragnostics is a precision medical discipline aiming to individualise patient targeted treatment. It aims at treating cancer by the systemic administration of a therapeutic radiopharmaceutical, which targets specific cells based on the labelling molecule. With the renewed interest in radiopharmaceutical therapy, the importance of accurate image quantification using iodine-123 (I-123) and iodine-131 (I-131), for dosimetry purposes, has been re-emphasised. Monte Carlo (MC) modelling techniques have been used extensively in Nuclear Medicine (NM), playing an essential role in modelling gamma cameras for the assessment of activity quantification accuracy, which is vital for accurate dosimetry. This thesis aimed to assess the accuracy of patient-specific I-131 dosimetry using hybrid whole-body (WB) planar-SPECT/CT imaging. The study was based on MC simulations of voxel-based digital phantoms, using the SIMIND MC code emulating the Siemens SymbiaTM T16 gamma camera. To achieve the aim, the thesis was divided into four objectives, (i) validating the accuracy of an energy resolution (ER) model, (ii) verifying the SIMIND setup for simulation of static, WB planar and SPECT images for I-123 with a low energy high resolution (LEHR) and a medium energy (ME) collimator and for I-131 with a high energy (HE) collimator, (iii) evaluating SPECT quantification accuracy for the three radionuclide- collimator combinations and (iv) assessing the accuracy of I-131 absorbed dose calculations for tumours and organs at risk, based on hybrid WB planar-SPECT/CT imaging. Methodology: The proposed ER model was fit to measured ER values (between 27.0 and 637.0 keV) as a function of photon energy. Measured and simulated energy spectra (in-air, in-scatter and a voxel-based digital patient phantom) were compared. The SIMIND setup was validated by comparing measured and simulated static and WB planar (extrinsic energy spectra, system sensitivity and system spatial resolution in-air and in-scatter), as well as SPECT (simple geometry sensitivity) results. Quantification accuracy was assessed in voxel- based digital simple and patient phantoms, using optimised OS-EM iterative reconstruction updates, calibration factor and recovery curves. Finally, using the true and quantitative activity data from I-123 and I-131 voxel-based digital patient phantoms, full MC radiation transport was performed, to determine the accuracy of the absorbed dose for I-131-mIBG radiopharmaceutical therapy. Results: The fitted ER model better simulated the energy response of the gamma camera, especially for high energy photopeaks, (I-123: 528.9 keV and I-131: 636.9 keV). The measured and simulated system energy spectra (differences ≤ 4.6 keV), system sensitivity (differences ≤ 6.9%), system spatial resolution (differences ≤ 6.4%) and SPECT validation results (difference ≤ 3.6%) compared well. Quantification errors less than 6.0% were obtained when appropriate corrections were applied. I-123 LEHR and I-123 ME quantification accuracies compared well (when corrections for septal scatter and penetration are applied), which can be useful in departments that perform I-123 studies and may not have access to ME collimators. Average I-131 absorbed doses of 2.0 ± 0.4 mGy/MBq (liver), 20.1 ± 4.0 mGy/MBq (3.0 cm tumour) and 22.6 ± 4.2 mGy/MBq (5.0 cm tumour) were obtained in simulated patient studies. When using a novel method of replacing the reconstructed activity distribution with a uniform activity distribution, eliminating the Gibbs artefact, the dosimetry accuracy was within 10.5%. Conclusion: Using the proposed fitted ER model, SIMIND could be used to accurately simulate static and WB planar and SPECT projection images of the Siemens SymbiaTM T16 SPECT/CT for both I-123 and I-131 with their respective collimators. Accurate quantification resulted in absorbed dose accuracies within 10.5%. The hybrid WB planar-SPECT/CT dosimetry method proved effective for personalised treatment planning of I-131 radiopharmaceutical therapy, with either I-123 or I-131 diagnostic imaging.Item Open Access Feasibility of tissue differentiation with multi-energy computed tomography: a Monte Carlo breast phantom study(University of the Free State, 2017-03) Van Eeden, Déte; Du Plessis, F. C. P.English: Dedicated breast CT is a new innovative way of imaging breast tissue without the limitations of overlapping anatomical features. It has been shown that the dose received by the patient is comparable to that of conventional mammography techniques. Further developments have led to the idea of a photon-counting detector that can be utilised in conjunction with breast CT. This will produce images with higher CNRs and will improve the detection of malignant masses. Other applications of multi-energy CT include image-based energy weighting and the differentiation of different tissues. The aim of this study was to explore the feasibility for tissue differentiation in breast tissue through the Monte Carlo simulation of a virtual multi-energy CT unit. The EGSnrc Monte Carlo code was used to simulate a virtual CT unit, similar to the Toshiba Aquillion LB 16 CT. The radiation source modelling code, BEAMnrc, was used to model the different components of the virtual CT. These components include the X-ray tube, suitable filters and beam-defining components such as collimators. A phase-space file was obtained consisting of all the particles generated by the different components. The energy spectrum of the Toshiba Aquillion LB 16 CT was approximated by the virtual CT using HVL measurements. The RMI electron density CT phantom was used to benchmark the virtual CT against the Toshiba Aquillion LB 16 CT. The phantom consists of several inserts with known electron densities that produce different CT numbers. A similar phantom was modelled with an in-house developed IDL program and used for the simulations. The reconstructed images were then used for the benchmarking of the HUs. This benchmarking ensured that the method used in this study produces a realistic model of a CT unit. Breast simulator software was used to model three breast phantoms consisting of different glandularities. The composition of the different breast tissues was taken from literature. The three phantoms were simulated at 20 keV up to 65 keV in 5 keV increments. All of the image reconstructions in this study was done with a filtered backprojection algorithm by using the OSCar reconstruction software. The CNRs of the different images obtained at different energies were assessed. Image-based energy weighting was investigated to further enhance the CNRs of the images by multiplying each energy bin with a specific weighting factor. The weighting factors were determined by a random number generator in an in-house developed IDL code. Good results were obtained with a 1.2-1.3 fold increase in the CNR. Further improvements were made by applying constraints to the weighting factors of the different energy bins. A new method was proposed to differentiate between different breast tissues by using the mass attenuation information from multiple energies. This technique showed promising results and can detect malignant tissue by using a single egs_cbct simulation. In conclusion, it is feasible to differentiate between different breast tissue types when using a multiple-energy CT unit. Better CNRs are obtained when utilising the information of the entire energy spectrum. This will lead to better tumour detection, even in dense breasts consisting of 89% glandular tissue.Item Open Access Sensitivity analysis of the intergral quality monitoring system® for radiotherapy verification using Monte Carlo simulation(University of the Free State, 2017-07) Oderinde, Oluwaseyi Michael; Du Plessis, F. C. P.Advanced radiotherapy (RT) techniques have improved the quality of radiation treatment. Notwithstanding, advanced RT techniques have generated complexities in their quality assurance (QA). Therefore, there is a huge interest to verify treatment plan data in real-time treatment. The Integral quality monitoring (IQM) system® (iRT Systems GmbH, Koblenz, Germany) is an independent real-time treatment verifying system which checks the integrity and validates the accuracy of the treatment plan data. The IQM also functions as a pre-treatment quality assurance tool for radiotherapy. The prototype system (IQM) is currently undergoing its beta testing, and contributions from researchers across the globe are pivotal to its integration into the clinical workflow. The IQM is a large wedge-shaped ionization chamber that is attached to the treatment head of the linear accelerator (linac) for signal measurement in real-time treatment. The aim of this innovative study was to determine how sensitive the IQM is for small alterations in the multileaf collimator (MLC) leaf positions using Monte Carlo (MC) simulation. The sensitivity of the IQM system is essential for its integration into clinical workflow. The MC simulation technique is an accurate dose calculation engine that could score dose in regions that seem complicated for physical measurement. A new component module (CM) called IQM was successfully developed using TCL/TK, and MORTRAN codes. The newly created CM was added-on to the BEAMnrc MC User code. Also, a linac source model of an Elekta Synergy linac equipped with an Agility 160-leaf MLC head was developed using the EGSnrc/BEAMnrc. Accurate MC calculations for percentage depth doses, lateral beam profiles, and relative output factors were benchmarked with physical measurements using the Gamma analysis criterion of 2%/2 mm. Characterised photon beams of 10 MV for 1 × 1 up to 30 × 30 cm2 fields using the BEAMnrc MC Code were simulated. Photon beam data stored in the phase space files after the source model simulations were calculated in a homogeneous water phantom using the DOSXYZnrc MC Code. For the square field sizes considered, MC dosimetry features (percentage depth doses and lateral beam profiles) passed the gamma (γ) index criterion of 2%/2 mm. MC calculations and physical measurements agreed to approximate local difference of 1.44% for relative output factors. This accurate source model is suitable for the sensitivity study. It also has the potential to be used for dose calculation in advanced radiotherapy treatment planning. The accurate source model with the IQM CM positioned with its central electrode plate fixed perpendicularly to the photon beam in subsequent simulations was used. The spatial integral dose in the air region of the IQM CM was calculated. The IQM MC dose was calculated for 1 × 1 up to 30 × 30 cm2 fields at 10 MV photon beams and then correlated with physical measurement of the prototype IQM system. Secondly, systematic positional errors of 1, 2 and 3 mm were subtracted and added to the whole MLC bank of 1 × 1, 3 × 3, 5 × 5 and 10 × 10 cm2 fields. Thirdly, the IQM signal response for 1, 2, 3, 4 and five leaves shifted out of a 5 × 5 cm2 field for positional error of 1, 2, 3, 5, and 10 mm was calculated. Fourthly, the signal response was calculated for segments along the gradient of the IQM CM for 3 × 3, 5 × 5 and 7 × 7 cm2 fields at 10 MV photon beams. Lastly, eleven segments (regular and irregular) were altered randomly within ±1, ±2 and ±3 mm regarding its individual leaf positions as defined at the isocentre. Sensitivity analyses of leaf positioning errors were studied by using the following techniques such as scatter plots, brute force, variance-based and standard regression coefficient. The normalised IQM signal increases with an increase in square field sizes for the MC calculation and the physical measurement. The IQM model is highly sensitive to alterations of 1 × 1 cm2 more than other fields considered. For the segments considered, the magnitude of the signal response decreased and increased when systematic positional errors were subtracted from and added to individual MLC leaves. An increase in numbers of leaves shifted out causes an increase in IQM signal response and an increase in the position of moving leaves causes a further increase in the IQM signal. The sensitivity of the IQM model increases along the gradient of the IQM up to a noticeable plateau. The sensitivity analysis techniques utilised in this study deduced that the IQM model is highly sensitive to leaf positions of small segments compared to large apertures. The newly developed IQM MC model can now serve as a basis for researchers that have an interest in dose monitoring and MLC calibration using the wedge-shaped ionization chamber. The IQM model shows a potential platform for further study on advanced radiotherapy quality control. Application of MC techniques to dose monitoring is authentic. It demonstrates that the MC radiation transport method is virtually unlimited when it comes to solving radiation transport and dose calculation challenges.