In this paper, we present an integrated system for real-time dose distribution calculation and treatment planning optimisation for brachytherapy of prostate cancer, with a special emphasis on the visual integration of the dosimetry and target images obtained from the open magnetic resonance system. This system involves a fast method to calculate dose distributions of multiple concurrent radioactive sources, based on the combination of elements from a database of pre-calculated dose distribution maps for single sources, combined linearly to provide the final dose distribution map. Simulated annealing, in conjunction with the inverse planning method, is used to determine the source dwell times at pre-selected locations in order to optimally irradiate thetumour while preserving the surrounding healthy tissues. This algorithm, implemented in FORTRAN, is integrated into a computer-assisted treatment planning tool, written in JAVA, using the runtime class and RMI API of Java. The whole system is now under clinical testing at the Geneva University Hospital.