Medical Physics

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Browsing Medical Physics by Advisor "Rae, William Ian Duncombe"

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    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.
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    ItemOpen Access
    Optimisation of delivery efficiency in prostate intensity modulated radiotherapy planning
    (University of the Free State, 2016-02) Fourie, Nicola Sieglinde; Muhammed, Omer Abdul-Aziz Ali; Rae, William Ian Duncombe
    English: Evidence that supports dose escalation for prostate cancer is growing and with Intensity Modulated Radiation Therapy (IMRT) higher conformal target doses can be delivered. With more segments and higher monitor units (MU’s), target conformity can be improved, however this results in longer delivery times, which makes it difficult to ensure accurate dose delivery, as intra-fractional as well as target movement plays an increasing role. Evidence from the literature indicates that secondary radiation-induced cancer risk is proportional to the beam-on time (thus the MU’s). Improvements in IMRT delivery efficiency while maintaining plan quality can be achieved by reducing the complexity of an IMRT plan. This can be done by changing the optimization parameters during the optimization process. Less “complex” prostate IMRT plans will require fewer MU’s by using less segments resulting in shorter delivery times and therefore reduced risk of secondary cancers. The goal of this study was to recommend a set of optimization parameter values that will improve the delivery efficiency of prostate IMRT treatment plan while maintaining plan quality. Fifteen clinical prostate IMRT plans (15 MV), already used for treatment, were re-optimized, using a XiO treatment planning system (TPS). Changes in total MU’s and segments were evaluated for changes in some of the optimization parameter values. Eleven optimization parameters (some of them used more than once with different values) were used to generate 15 new IMRT combination plans (ICP’s) for each patient for both 6 and 15 MV, resulting in 450 plans being assessed. One parameter was changed at a time while all other variables were kept constant. Plan quality was evaluated in terms of four variables: MU, number of segments, homogeneity index and conformity index while the delivery efficiency was evaluated in terms of delivery time. To our knowledge no time delivery model has been proposed for a Siemens® ARTISTETM Linear Accelerator (Linac). Using the principles given in the literature we derived such a time delivery model by adding the radio frequency wave component and Multi Leaf Collimator delay time. K-means clustering was then used to analyse the data in terms of the five variables and the top 10 ICP’s in 3 patients in terms of a faster more conformal, delivered plan were identified. To confirm the delivery efficiency and accuracy, the fluences of these top 10 ICP’s were measured on a Siemens® ARTISTETM Linac with the step and shoot method and compared to the treatment planning system’s fluences. The evaluation criteria chosen were 3% and 3 mm, distance to agreement. A 3 dimensional dose volume histogram program was used to determine the percentage pass rates on the planned target volumes and the organs at risk. The optimization parameters such as the minimum MU’s per segment, intensity level, minimum segment size and minimum segment area; demonstrated the greatest influence on the total number of segments, while the total MU’s was most greatly influenced by the filters and intensity level optimization parameter. Controversy exists regarding which energy should be used, 6 MV or 15 MV, when treating prostate cancer. Both energies were considered here during the optimization process and it was concluded that the optimization parameters are not greatly influenced by the beam energy. However, it was seen that beam arrangement has an influence on optimization parameter behaviour. A limitation of this study is that the beam angle distribution was not investigated. Thus recommendations could be made in terms of which ICP demonstrated the most improved delivery efficiency of a prostate IMRT treatment plan while maintaining plan quality. The optimisation parameter which was introduced to the optimization process was a General High filter. Gaining knowledge about the behaviour of the optimization parameters during optimization makes it easier to advise and assist treatment planners preparing complex IMRT plans.
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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. ___________________________________________________________________