Development of a brachytherapy treatment planning module for cervix cancer utilising biological dose metrics
Van der Walt, Hester Catharina
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The contouring uncertainties associated with the use of computed tomography imaging for brachytherapy planning creates the need to investigate an alternative planning method for Image-Guided Adaptative Brachytherapy. This alternative method needs to be more robust against imaging and contouring uncertainties compared to the original GEC-ESTRO prescription regarding dose-volume histogram criteria. This study evaluates the utilisation of biological dose metrics (equivalent uniform dose (EUD)) during Image-Guided Adaptive Brachytherapy (IGABT) treatment planning and optimisation. A retrospective planning study was conducted. The eighteen patients that were included in the planning study received CT-based Image-Guided Brachytherapy (IGBT) in combination with external beam radiotherapy (EBRT) between 2014 and 2015. A novel biological optimisation model was developed and used to efficiently and automatically optimise brachytherapy (BT) plans by utilising either dose-volume, biological metrics/indexes or both for fast treatment plan generation. The module was refined to allow forward and inverse planning and optimisation of combined interstitial and intracavitary brachytherapy. Additionally, the utilisation of OAR total dose constraints during the planning process was applied and evaluated. The results of the study showed that the inverse optimisation tool could produce better target volume coverage compared to the manual optimisation tool. In some cases, the inverse optimisation tool led to higher OAR doses; however, the values recorded were still within set constraints. When comparing the conventional and biological planning methods, the biological planning produced superior CTV-T doses and dose distributions within the CTV-T. The inverse biological approach reported significantly higher average CTV-THR D98 % , D100 % values and CTV-TIR EUD, D90 % D98 % and D100 % values compared to inverse conventional planning approach. With this, the inverse biological approach also had the ability to record significant lower average bladder wall EUD and D0.1cm3D0.1cm3 values. Even though the inverse conventional planning approach reported significant lower average small bowel D2cm3 values compared to the IBG approach, both approaches were still well below the D2cm3 hard constraint of 75 Gy. Dose escalation was achieved in the CTV-T with a reduction in OAR dose with the combination of interstitial/intracavitary brachytherapy. It was concluded from the study that the incorporation and utilisation of biological metrics, which incorporates the entire dose distribution in the organ of interest, is the preferred approach when compared to the conventional physical dose-volume approach.