Doctoral Degrees (Medical Physics)

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Browsing Doctoral Degrees (Medical Physics) by Author "Rae, William Ian Duncombe"

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    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.
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    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. ___________________________________________________________________