The invention relates to a method of characterizing the cellular reaction of a cell sample to ionizing radiation, the cell sample having been obtained from cells taken from a non-irradiated or slightly irradiated region of a patient, and the method involving the following steps: (i) amplifying the sampled cells, the amplified cells constituting the cell sample； (ii) determining on the cell sample the mean number of nuclear foci obtained with at least two of the markers pH2AX, pATM and MRE11 at observation times t (these mean numbers being designated NpH2Ax(t), ΝpΑΤΜ(t), NMRE11(t), respectively), the observation times t being the time t=0 min (designated t0, non-irradiated state) and at least one observation time selected from t = t1, t2, t3 and t4 following irradiation with an absorbed dose D； and (iii) determining at least one parameter selected from the group consisting of: the degree of severity of the post- radiotherapy tissular reaction according to the CTCAE classification, by using at least the mean numbers NpH2Ax(t) and NpH2Ax(tx), where tx is either t4 (preferable) or t3； and the radiosensitivity of the sample, using at least the mean numbers NpH2Ax(t), NpATM(t) and NMRE11(t)； and, in which method: t4 is a fixed value representing the time taken for the DNA breakage rate to reach its residual value； t3 and t2 are fixed values representing the time at the end of which approximately 25 % and 50 %, respectively, of the double-strand breakages have been repaired in the control cells from radioresistant patients； and t1 is a fixed value representing the time at the end of which the number of double-strand breakages recognized has reached its maximum in the control cells from radioresistant patients.
