1. A system for obtaining at least one patient specific threshold for discrimination among different tissue types within a patient, the system comprising: an optical probe configured to obtain light measurements at one or more positions in tissue of a patient; a means for confirming said optical probe is positioned to obtain light measurements at one or more positions in patient tissue of a first tissue type; and a console arranged configured to: spectrally analyze light measurements obtained by the optical probe at said one or more positions in patient tissue of the first tissue type, calculate predetermined spectral characteristics for the patient tissue of the first tissue type derived from fits of the spectrally analyzed light measurements obtained by the optical probe at said one or more positions in patient tissue of the first tissue type define patient specific first tissue type data points in a score plot of a multivariate statistical analysis to form a patient specific first tissue type class as a first tissue type class on a basis of the calculated predetermined spectral characteristics for the patient tissue of the first tissue type, define a first patient specific threshold on the score plot for the first tissue type class by using (1) the calculated predetermined spectral characteristics for the patient tissue of the first tissue type as a reference tissue of the first tissue type class and (2) intra-patient a priori knowledge of the predetermined characteristics for the first tissue type and/or other tissue types relative to the first tissue type, spectrally analyze further light measurements obtained by the optical probe at another position in patient tissue, calculate the predetermined spectral characteristics for the further light measurements derived from fits of the spectrally analyzed further light measurements obtained by the optical probe at said another position in patient tissue, define data points for the further light measurements in the score plot on a basis of the calculated predetermined spectral characteristics for the further light measurements, test whether the data points for the further light measurements fall outside the first patient specific threshold in said score plot, and define a patient specific second tissue type class as a second tissue type class on the basis of the calculated predetermined spectral characteristics for the further light measurements if the data points for the further light measurements fall outside the first patient specific threshold in said score plot.