Method for characterizing the radiosensitivity of a patient's cell sample to ionizing radiation, said cell sample having been obtained from cells taken from a patient, in which method: (a) target nucleated cells constituting the cell sample are isolated; (b) said cell sample is irradiated before or after step (a) with an absorbed dose D of ionizing radiation; (c) the cytoplasmic and nuclear fractions are recovered from the cell sample; (d) the following are determined on the cytoplasmic and/or nuclear fractions of said cell sample: the quantity of substrate phosphorylated by the activated ATM protein, which is a target protein phosphorylated by the ATM kinase, at at least one observation time t between t1 times 1.2 and t3, preferably at at least one observation time t2 after irradiation with an absorbed dose D; and/or the kinase activity by quantifying phosphorylated pATM at at least one observation time t selected from t = t1, t2; (e) at least one parameter is determined, selected from the group formed by: - the degree of severity of the patient's post-radiotherapy tissue reaction according to CTCAE classification, using at least the mean number pATM max nuc (t); - the radiosensitivity of the cell sample, using at least the mean number pATM max nuc (t); and/or • t1 is a fixed value which represents the time after which the number of double-strand breaks recognized reaches its maximum in control cells obtained from radioresistant patients; • t2 is a fixed value which represents the time after which approximately 50% of the DSBs are repaired in control fibroblasts obtained from radioresistant patients and the time after which 30% of the DSBs are repaired in control lymphocytes or lymphoblasts obtained from radioresistant patients; • t3 is a fixed value which represents z times t2, where z is between 1.1 and 8, and preferably between 1.2 and 6. (a) target nucleated cells constituting the cell sample are isolated; (b) said cell sample is irradiated before or after step (a) with an absorbed dose D of ionizing radiation; (c) the cytoplasmic and nuclear fractions are recovered from the cell sample; (d) the following are determined on the cytoplasmic and/or nuclear fractions of said cell sample: the quantity of substrate phosphorylated by the activated ATM protein, which is a target protein phosphorylated by the ATM kinase, at at least one observation time t between t1 times 1.2 and t3, preferably at at least one observation time t2 after irradiation with an absorbed dose D; and/or the kinase activity by quantifying phosphorylated pATM at at least one observation time t selected from t = t1, t2; the quantity of substrate phosphorylated by the activated ATM protein, which is a target protein phosphorylated by the ATM kinase, at at least one observation time t between t1 times 1.2 and t3, preferably at at least one observation time t2 after irradiation with an absorbed dose D; and/or the kinase activity by quantifying phosphorylated pATM at at least one observation time t selected from t = t1, t2; (e) at least one parameter is determined, selected from the group formed by: - the degree of severity of the patient's post-radiotherapy tissue reaction according to CTCAE classification, using at least the mean number pATM max nuc (t); - the radiosensitivity of the cell sample, using at least the mean number pATM max nuc (t); and/or • t1 is a fixed value which represents the time after which the number of double-strand breaks recognized reaches its maximum in control cells obtained from radioresistant patients; • t2 is a fixed value which represents the time after which approximately 50% of the DSBs are repaired in control fibroblasts obtained from radioresistant patients and the time after which 30% of the DSBs are repaired in control lymphocytes or lymphoblasts obtained from radioresistant patients; • t3 is a fixed value which represents z times t2, where z is between 1.1 and 8, and preferably between 1.2 and 6. - the degree of severity of the patient's post-radiotherapy tissue reaction according to CTCAE classification, using at least the mean number pATM max nuc (t); - the radiosensitivity of the cell sample, using at least the mean number pATM max nuc (t); • t1 is a fixed value which represents the time after which the number of double-strand breaks recognized reaches its maximum in control cells obtained from radioresistant patients; • t2 is a fixed value which represents the time after which approximately 50% of the DSBs are repaired in control fibroblasts obtained from radioresistant patients and the time after which 30% of the DSBs are repaired in control lymphocytes or lymphoblasts obtained from radioresistant patients; • t3 is a fixed value which represents z times t2, where z is between 1.1 and 8, and preferably between 1.2 and 6.