Doctoral Degrees (Medical Physics)

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  • ItemOpen Access
    Accuracy of lutetium-177 SPECT activity quantification and patient-specific dosimetry: a Monte Carlo study
    (University of the Free State, 2021) Ramonaheng, Keamogetswe; Van Staden, Johannes A.; Du Raan, Hanlie
    𝑬𝒏𝒈𝒍𝒊𝒔𝒉 The goal of radiopharmaceutical therapy (RPT) is to deliver the maximum dose to cancerous tumours while sparing healthy tissue. Ideally, the radiopharmaceutical should accumulate in the tumorous tissue and the radiation entirely absorbed for tissue destruction; however, this is not the case. Dosimetry strives to balance the efficacy of delivering the maximum dose to the tumour cells with minimal toxicity to the healthy tissue. Patient-specific dosimetry offers the potential for RPT to reach its full potential as a powerful precision-based treatment. Efforts for patient-specific dosimetry remain a challenge due to the steps involved in the clinical dosimetry workflow. SPECT/CT imaging allows the estimation of the bio-kinetic distribution of the radiopharmaceuticals with good precision, which is required for accurate dosimetry. Different dosimetry software is available (commercial and non-commercial), but these should be benchmarked before being used. Optimising the imaging process for activity quantification and dosimetry in the NM discipline is essential, and Monte Carlo (MC) techniques have been used successfully in this endeavour. Furthermore, full MC dosimetry gained wide acceptance as the gold standard and the most accurate means for patient-specific dosimetry. MC simulations offer the advantage of having a gold standard against which the dosimetry can be benchmarked, and the dosimetry accuracy evaluated. Therefore this thesis aimed to assess the accuracy of 177Lu SPECT activity quantification and patient-specific dosimetry using MC simulations. The focus was on the bio-distribution of 177Lu- DOTATATE due to its clinical relevance in RPT of patients with metastasised neuroendocrine tumours. The kidneys are the dose-limiting organs for RPT with 177Lu-DOTATATE. Studies have shown kidney doses to vary significantly between patients. Given the relation between tumour absorbed dose and tumour reduction, for a complete efficacy evaluation, absorbed doses should be determined not only for the kidneys but, where possible, extended to the tumours. This study incorporated voxel-based phantoms generated from phantom and patient CT data to perform virtual image-based activity quantification and dosimetry using MC simulations. The voxel-based phantoms were modified to include spherical structures mimicking tumours. The first objective of this study was to validate a model of the Siemens Symbia T16 dual-head SPECT/CT gamma camera available in our clinic using the SIMIND MC program for 177Lu imaging. The validation was achieved by comparing experimental and simulated gamma camera performance planar and SPECT criteria tests. The results were in good agreement and provided adequate confidence that SIMIND could emulate the Symbia T16 successfully and be used for further investigations of 177Lu SPECT/CT image quantification. The second objective investigated the effect of sphere and cylinder calibration factor (CF) geometries and their corresponding recovery coefficients (RCs) on the quantification accuracy of 177Lu SPECT images using MC simulations. The investigations were performed using geometries of a cylindrical, an anthropomorphic torso, and patient-specific phantoms. The quantification accuracy was evaluated for tumours and the kidneys. The results demonstrated that 177Lu SPECT quantification accuracies compared favourably for sphere-based and cylinder-based CF and RC combinations when all SPECT corrections were applied. The absolute quantification accuracy of ≤ 3.5% compared well to literature findings and complied with the 5% requirements for accurate dosimetry. The third objective of the thesis aimed to compare the accuracy of the absorbed doses computed with the software LundADose and OLINDA/EXM 1.0 using three patient-specific voxel-based phantoms. The dosimetry accuracy was assessed by comparing the computed doses to the “true” activity images combined with full MC dosimetry to define the gold standard. The accuracy between LundADose (6.6%) and OLINDA/EXM 1.0 (8.1%) was comparable. The ≤ 10% dosimetry accuracy suggested that the software platforms approximated the true dose estimates and advocated for the dosimetry accuracy to be reliable. ___________________________________________________________________
  • ItemOpen 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.
  • ItemOpen 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.
  • ItemOpen Access
    Accuracy of iodine-131 activity quantification and dosimetry for three-dimensional patient-specific models
    (University of the Free State, 2019-03) Ejeh, John Enyi; Van Staden, J. A.; Du Raan, H.
    Iodine-131 (131I) therapy of thyroid related and other diseases is limited by critical organ toxicity. Therefore, accurate activity quantification and dose calculation are important to optimise dose to tumours while limiting dose to critical organs. The aim of this study was to evaluate the accuracy of 131I activity quantification and dosimetry for three-dimensional (3-D) patient-specific models. Retrospective patient Computed Tomography (CT) data were segmented to create clinically realistic patient 3-D voxel-based models. These were used to simulate Single Photon Emission Computed Tomography (SPECT) data with a Monte Carlo (MC) simulation software, which was validated against physical measurements. The simulated SPECT data were reconstructed using an ordered-subsets expectation maximization (OS-EM) algorithm which includes scatter correction, CT-based attenuation correction, and 3-D collimator-detector response compensation. Predetermined recovery coefficients were used to compensate for partial volume effects. Image counts were converted to activity by using a predetermined calibration factor. The patients’ reconstructed activity maps and density maps were used to perform 3-D dosimetry with the MC program, LundADose. LundADose calculated mean tumour and organ absorbed doses were compared with OLINDA/EXM calculated mean absorbed doses using statistical analysis. Validation of the simulation software resulted in a percentage difference of -6.50 % between the measured and simulated extrinsic energy resolution at the 131I peak energy of 364 keV and - 18.57 % error for the measured and simulated intrinsic energy resolution. The measured and simulated FWHM and FWTM of the camera for system spatial resolution had percentage differences of -7.41% and -7.38 % and an error of -1.50 % and -2.6 % for system sensitivity and collimator septal penetration fraction. SPECT activity quantification was evaluated by comparing the true tumour activities defined for the patient models with the quantified activities obtained from the models’ reconstructed SPECT images. The quantification error for the studied patient models was < 9.0 % and < 5.1 % for 3.0 and 6.0 cm spherical tumours situated in the lungs (mean values were 3.9 ± 3.3 % and -1.6 ± 1.9 %). The error for the two tumours in the liver was < 11.2 % (mean values of 7.7 ± 3.9 % and 8.4 ± 2.9 %). The mean percentage differences between the mean absorbed doses calculated by LundADose and OLINDA/EXM for the left lung, right lung, liver, 3.0 cm ‘tumour’ and 6.0 cm ‘tumour’ were comparable. These mean percentage differences were -2.23 ± 1.98 %, -3.06 ± 1.67 %, 1.31 ± 4.15 %, -28.44 ± 18.36 %, and -5.10 ± 2.87% for the listed organs and tumours when the 3.0 cm tumour was located in the lung and the 6.0 cm tumour in the liver. For the scenario where the 3.0 cm tumour was positioned in the liver and the 6.0 cm tumour in the lung, the corresponding results were -2.84 ± 3.42 %, -1.49 ± 2.68 %, 3.97 ± 4.12 %, -28.80 ± 5.05 %, - 8.21 ± 17.06 %. The SIMIND MC model of the gamma camera was accurately validated with good agreement between results calculated from the physical measurements and simulation. Good accuracy of 131I activity quantification and 3-D dosimetry was found for 3-D patient-specific models. Statistical analysis of the results of the comparison of LundADose and OLINDA/EXM showed that the two dosimetry programs were strongly correlated with R2 values ranging from 0.85 to 1.00 for the mean absorbed dose in the various organs and tumours. Furthermore, the two (MC and MIRD) methods were found to agree well using Bland-Altman analysis of the dosimetry results. For 131I, activity quantification and dosimetric accuracy better than 10 % were achieved using state-of-the-art hybrid equipment and sophisticated correction methods for image degrading factors.
  • ItemOpen Access
    Segmentation and quantitative characterization of breast masses imaged using digital mammography
    (University of the Free State, 2018) Nkwenti, Sussan Acho; Rae, William Ian Duncombe
    𝑬𝒏𝒈𝒍𝒊𝒔𝒉 Breast cancer is the leading cause of cancer death among women. Screening Mammography is the most effective method currently available for early detection of breast cancer. When breast cancer is detected at an early stage the prognosis is good because the tumour is smaller and more often well-differentiated, and less likely to have spread to regional lymph nodes. Computed radiography and direct digital detector mammography imaging systems provide a wide dynamic range for proper display of different densities of breast tissue areas. Their response over a wide range of X-ray intensities is linear; consequently, small differences between the attenuation coefficients of breast structures over a wide range of densities are clearly displayed. This includes the low signal areas associated with high densities found within tumours. Some masses infiltrate the surrounding breast tissue hence they exhibit ill- defined and intensity inhomogeneous boundaries with rough contour, while other masses exhibit well-defined edges and in most cases they possess smooth, round or oval shapes with macro-lobulations. The morphologic features of a mass such as its shape, margin and density give a clue to its benign or malignant nature. This study investigates and quantifies the changes in shape-based descriptors due to changes in the location of the initial level set contour in region based active contour models in delineating mammographic masses and proposes new methods to eliminate contour leakage and contour traps in active contour segmentation models which are due to intensity inhomogeneity within tumours and boundary regions of tumours. Furthermore, the study proposes a contextual region of interest model to assess the variation of texture features from the core to its periphery of biopsy proven malignant masses as a concept of tumour modelling in mammography and also the variation of texture features between grade 2 and grade 3 masses as a concept of tumour grading in mammography with texture analysis. ___________________________________________________________________
  • ItemOpen 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.
  • ItemOpen 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.
  • ItemOpen Access
    Development and evaluation of a spect attenuation correction method using an open transmission source and scatter correction
    (University of the Free State, 2011-06) Van Staden, Johannes Abraham; Du Raan, H.; Van Aswegen, A.
    Abstract not available
  • ItemOpen Access
    An equivalent uniform dose-based class solution for cervical cancer radiotherapy
    (University of the Free State, 2014-06) Shaw, William; Rae, William Ian Duncombe; Alber, Markus Lothar
    English: Cervix cancer radiotherapy treatment consists of external beam radiotherapy (EBRT) and brachytherapy (BT). Currently there exists no method to combine the dose of both modalities in a single dose value or dose distribution. This study derived a method to use the equivalent uniform dose (EUD) concept as a worst case dose estimate for both modalities, and the combination thereof. The EUD was used as dose evaluation tool in clinical brachytherapy planning of 10 patients that received conservative organ at risk (OAR) toxicity avoidance treatment plans. OAR EUD dose constraints were also derived for brachytherapy treatment planning so as to be equivalent to the Gyn GEC-ESTRO guidelines for cervix cancer brachytherapy based on a population of 20 patients receiving 5 high-dose-rate image guided brachytherapy treatments each. Furthermore, a method to escalate tumour dose without increasing OAR dose was investigated using the EUD as a safeguard against OAR over-dosage and exploiting the effects of fractionation radiobiologically and by organ geometry variations. The EUD was also used as an external beam IMRT evaluation tool to calculate suitable planning target volume (PTV) margin sizes for treatment plan optimization and as a quick cumulative dose computation to enable on-line and off-line image guided adaptive radiotherapy (IGART). This study utilizes the underlying mathematical properties of the EUD to act as a method for determining a worst case dose estimate for tumours and OARs. The method is accurate and reliable and easy to use. OAR dose constraints for brachytherapy treatment planning based on EUD prescription were derived and they compare well with existing Gyn GEC-ESTRO recommended methods and constraints. The safety of the EUD as a worst case dose estimate motivates the use thereof in fractionation compensation based treatment planning that strives to maximize OAR dose to a fixed constraint level and maximize tumour dose at no extra toxicity cost. The EUD derived external beam planning margins also corresponded well with the published margin recipes, but showed that margin recipes potentially overestimate the required margin size and that PTV dose levels could be reasonably lower in some cases compared to the CTV dose level and not lead to tumour under-dosage. The EUD is also an effective 4D dose evaluation and planning tool for IGART and can be used to ensure adequate total dose is delivered in a mobile and deforming tumour without overdosing the OARs. The quick and reliable application of this method is its biggest attribute. The mathematical properties of the EUD open the possibility to determine a worst case estimate of cumulative dose in different treatment modalities and when they are used in combination. The application of this estimate can be extended to safe tumour dose escalation in both image guided adaptive brachytherapy (IGABT) and IGART and their combination.
  • ItemOpen Access
    Assessment of factors affecting accuracy of standardised uptake values in positron emission tomography
    (University of the Free State, 2015-01) Du Toit, Petrus Daniel; Du Raan, H.; Rae, W. I. D.; Visvikis, D.
    English: Positron emission tomography (PET) is an imaging method that uses tracers labelled with positron emitting isotopes for the monitoring and evaluation of in vivo molecular processes. Semi-quantitative determination of tracer uptake in a lesion is accomplished by calculating the standardised uptake value (SUV), an index that represents the amount of uptake in a given volume-of-interest (VoI) in relation to the average uptake throughout the body. The SUV is influenced by biological and physical factors that determine the uptake or detectability of the tracers which may result in false results. Changes in SUV of small lesions or lesions with low activity uptake cannot be determined with enough certainty and precision to be used for decision-making and it is therefore necessary to investigate the factors affecting the SUV. The aim of this study was to assess the relative importance of the physical factors that affect the accuracy of a single SUV measurement using Monte Carlo modelling. Phantom studies were performed to determine the influence of the partial volume effect due to spatial resolution using a PET scanner. Comparative Monte Carlo simulations were performed on a computer cluster using a voxelised version of the same phantom. The XCAT anthropomorphic phantom was used to assess the influences on SUV in a human-like configuration and was set-up to simulate movement in the thorax during breathing. SUVs were calculated using simulations of the phantom in 2D and 3D modes to assess the influence of the partial volume effect by variation of the size of the lesions, by variation of the contrast ratios and by placing the lesions in different areas in the lungs during. Influence of activity from outside the field-of-view (FoV) was also assessed as well as the impact the various coincidence types have. Statistical methods were used to compare the difference in data for statistical significance. It was found that the partial volume effect was present when evaluating the SUVs of the activity in the spheres of the phantom when scanned on a PET/CT scanner as well as when performing Monte Carlo simulations. Statistically there were no significant differences between the two scanning modes. The mean SUV increased as the voxel sizes became smaller. The choice of matrix influenced the amount of partial volume effect. The relative contributions of true-, scatter- and random coincidences demonstrated that the true coincidences were the major contributor when assessing the data from this phantom. The relative contribution of the trues-to-total coincidences decreased with a decrease in lesion size and contrast ratio whereas the relative contributions of the scattered- and random coincidences increased. The contributions of scatters and randoms increased during the 3D acquisition mode compared to 2D mode. The contribution of the trues-to-total coincidences decreased with an increase in VoI size and consequently caused a decrease in the mean SUV. The location of the lesion made a difference in SUV when the same size lesions are compared to each other. Apical lesions experienced the least amount of motion during breathing, were distorted less and had the least amount of variation in SUV. By moving the phantom partly outside the FoV, significant effects on the SUVs of objects still inside the FoV were found. An increase in the SUVs was observed when the true coincidences were used for the calculation. A decrease in true SUVs was found at the right basal lesion. In conclusion, partial volume effects play a significant role when determining the SUV of objects based on their size and contrast ratio; the location of pulmonary lesions affects SUV calculation during breathing; and activity outside the field-of-view of the scanner contributed to a change in SUV in particular to the central and basal regions of the lung.