In several places in the world, such as Petten and Delft in the Netherlands, investigations are in progress in the fight against certain types of cancer with Boron Neutron Capture Therapy. The basic idea is very simple: boron is loaded only into the cancer cells, using a special drug, after which the patient is irradiated with neutrons. Especially slow or low energy neutrons that collide with the boron have a high probability to induce a highly energetic reaction which can destroy the cancer cell nucleus, causing the death of the cancer cell. The research performed by the Delft University of Technology in cooperation with the Joint Research Centre of the European Commission in Petten is focussed on finding the ideal velocity of the neutrons coming from the source before they slow down in the patient and arrive in the neighbourhood of the tumour. A mathematical algorithm is applied to calculate the neutron transport from the tumour to the neutron source, i.e. backwards. As a result, one can determine the optimal velocity of the neutrons at the source location. This method decreases significantly the time to obtain a treatment plan, for example, for a patient suffering from metastases of melanoma in the brain. In addition, work has also been performed to find the optimum set of source neutron velocities in order to obtain the same number of slow neutrons everywhere in a liver, full of metastases, that will be treated (irradiated) after having been temporarily removed form the patients body. Finally, it was investigated what the ideal velocity of the source neutrons is, when taking into account the different sensitivities of the overlying tissues during an irradiation. The result of the research is that the efficiency of BNCT can be increased by also having neutrons available, besides the neutrons already used, which move 200 times slower.