The present invention relates to an in vitro method for assessing the risk of developing side effects after ionizing radiation treatment in a prostate cancer subject, comprising the steps of a)measuring radiation induced T-lymphocytes apoptosis in a sample of the subject; b)determining the presence of urinary toxicity in the patient prior to application of ionizing radiation, and optionally of at least one other clinical parameter, disease parameter or ionizing radiation treatment parameter from the patient, and c)combining the value of the at least one biochemical marker measured in step(a) and a value associated with the at least one clinical parameter, disease parameter or ionizing radiation treatment parameter determined in step (b) in a mathematical function to obtain an end-value.