Numerical modelling of bladder filling and prostate positioning in modern radiotherapy

The human bladder is an important organ within the pelvic system of the human body. It routinely collects urine excreted by the kidneys prior to disposal via urination causing significant variation in the shape and volume of the bladder. The mechanics of the filling process directly affects the function of the bladder and its interaction with the surrounding organs, in particular the position of the prostate that can influence the diagnosis and planning of radiotherapy treatments of prostate cancer. The success of prostate radiotherapy depends on the delivery of high doses of radiation to a defined tissue volume with a high degree of positional accuracy. A detailed study is required to establish the volume and shape change of the bladder during the course of the radiotherapy treatment, over a long period of time, and to develop an effective modelling tool to simulate this complex mechanical process to provide guidance to diagnosis and treatment planning. In this work, systematic data of the bladder dimension are presented based on measurement of Computed Tomography (CT) images of a group of 10 patients with prostate cancer, over the course of radiotherapy treatment. A method of measuring the bladder wall distance in the anterior-posterior (AP), superiorinferior (SI) and left-right (LR) directions is implemented. Systematic data obtained from scanned images taken on the transaxial, sagittal and coronal planes are analysed and compared to assess the potential influence of the scanning sequence on the shape and size of the bladder. The variation of the bladder dimensions, volume and shape were analysed between different arms for the same subject and the upper limit of bladder volume variation were determined. The relationship between some key dimensional parameters (such as wall distance and aspect ratio) with the volume change is established. Key systematic results of prostatic displacement are determined, and inter subject difference of prostate positional change is analysed. The process of bladder filling and potential inter and intra subject difference and their influence on the diagnosis and planning process are discussed. A systematic numerical modelling method based on MR images has been developed to simulate the mechanics of bladder filling and its effects on the position of the prostate. A new approach for constructing detailed three dimensional (3D) Finite Element (FE) models that were specific to each patient were developed using multiple magnetic resonance (MR) images taken in three different planes (transaxial, sagittal, and coronal). Detailed sensitivity studies have been performed in comparing the 2-D and 3-D model, mesh size, materials models and parameters. The overall model of bladder deformation was compared with repeated images of the filled bladder that were obtained using computed tomography (CT) to validate the FE models. FE bladder deformation was found to be comparable to repeated CT images of the same patient, thus validating the FE models and the approach used in this work. The relationship between bladder deformation and its volume change has been established. The numerical results showed that the bladder dimensions increased linearly with its volume and the predicted coefficients is comparable to some published clinical results. The movement of the prostate in the anterior-posterior (AP), superior-inferior (SI), left-right (LR) directions, as well as its rotational movement, that is associated with changes in bladder volume was predicted. The numerical results showed that the movement of the prostate in the AP and SI directions (3-4mm and 1.S-4mm) was much greater than that in the LR direction (O.5-1mm). The scale of the movement of the prostate with the increase in bladder volume varied considerably among subjects. The numerical predictions were compared with published clinical data and the significance of the results in medical application is discussed. The work showed that subject specific Finite Element modelling could potentially provide crucial guidance for the medical practice on the treatment planning process of the prostate cancer.