The methodology we have developed to study dose distribution outside the target volume during external beam radiotherapy allows us to determine the dose received by the patient arising from leakage radiation and scattered radiation from both the head of the treatment machine and from the treatment room. It also allows us to evaluate the dose due to photon scattering in the patient by means of a dedicated 3-D algorithm permitting computations for the whole body of the patient and taking into account height, sex and anatomical data at the time of the treatment, dosimetric data and lung heterogeneity parameters. This methodology offers solid criteria for recommendations concerning radiation protection.