The thermal column of the TRIGA reactor in Mainz is being used very effectively for medical and biological applications. The BNCT (boron neutron capture therapy) project at the University of Mainz is focussed on the treatment of liver tumours, similar to the work performed at Pavia (Italy) a few years ago, where patients with liver metastases were treated successfully by combining BNCT with auto-transplantation of the organ. Here, in Mainz, a preclinical trial has been started on patients suffering from liver metastases of colorectal carcinoma. In vitro experiments and the first animal tests have also been initiated to investigate radiobiological effects of radiation generated during BNCT. For both experiments and treatment, a reliable dosimetry system is necessary. From work elsewhere, the use of alanine detectors appear to be an appropriate dosimetry technique.