The method (100) involves modeling an elementary theoretical action volume of an optical fiber by an elementary theoretical action volume of a cylinder having a swinging radius and length corresponding to an insertion length of the fiber. The swinging radius is determined by correlation of measured volumes of effectively necrosed zones following clinical tests carried out on different patients by utilizing a photosensitive substance associated with the fiber, using theoretical volumes calculated from parameters utilized in the tests and the elementary theoretical action volume of the fiber. The parameters are number of optical fibers, position of the fibers with respect to an external brachytherapy grid and insertion lengths of the fibers. Independent claims are also included for the following: (1) a computer implemented method for assisting scheduling treatment for a patient by photodynamic therapy (2) a computer program product comprising instructions for performing a method for modeling a swinging radius of an optical fiber utilized in a treatment for a patient by photodynamic therapy (3) a computer program product comprising instructions for performing a method for assisting scheduling of treatment for a patient by photodynamic therapy.
