An improved radiation therapy planning procedure is suggested. The procedure comprises the steps of specifying and determining the absolute grade of cell degeneracy by in-vitro tests, whereby marker(s) indicative for specific cell degeneracy are detected and quantified, establishing a biology-based segmentation of areas with similar grade of relative cell degeneracy and applying the absolute grade of cell degeneracy to the biology-based segmentation data, thereby establishing an improved radiation therapy planning procedure. Moreover, the present invention suggests a system for an improved radiation therapy planning procedure and its use in procedures of diagnosis and/or therapy management of cancer.