The ever increasing number of solid tumours and revival of radiotherapy as the method of choice of locoregional treatment in particular in combination with surgery in curative treatment of not advanced diseases leads to the fact that the specialist in internal medicine is faced with early as well late postirradiation changes which may play a part in the differential diagnosis in internal medicine The radiation effects on the living organism can be divided into those conditioned by cellular losses (deterministic) and structural DNA changes in surviving cells (stochastic). Both types of changes must be taken into account in relation to the administered radiation dose, size and site of the irradiated volume, fractionation and time interval after radiation. Modern radiotherapy uses for axternal irradiation as well as for brachytherapy specialized equipment and facilities which make optimalization of treatment possible so that the defined clinical target volume is irradiated homogeneously by a defined dose with minimal damage of surrounding tissue. Contemporary oncological multimodal therapy is ever more successful, patients survive in a satisfactory clinical condition without signs of neoplastic disease for increasing periods of time. In the differential diagnosis in the subsequent development of every patient it is important to record in the case-history whether the patient was irradiated and when and it must be taken into consideration that late postirradiation changes are permanent and develop even several years after completed radiotherapy. In case of doubt, one way leading to a correct diagnosis (and thus also treatment) is consultation with the radiotherapist who treated the patient and who should follow him up permanently after completed treatment.

        Key words: Complications of radiotherapy - Deterministic effects - Stochastic effects - Medical complications of radiotherapy