[Time factors factors in radiotherapy. Introduction (author's transl)]

The role of time factors, the fractioning and spread of irradiation, continue to occupy an important place in conference, symposia and meeting of radiotherapists and our own Society has followed this tradition by devoting to the subject this session of the Journées de Radiologie 1975. The problem is a constant feature of everyday clinical practise. Whilst the majority of radiotherapists are prudently attached to the traditional method of 5 sessions per week, they are frequently forced to change this rhythm for pathological reasons, the personal convenience of the patient or problems with apparatus. The association of transcutaneous and curietherapeutic irradiation adds new parameters : the rate of the curie-therapy, the relative contribution of the 2 methods of irradiation, their sequence and the interval which separates them. Poor results of radiotherapy in certain situations indicates the possible value of determining whether other methods of the distribution per session and in time might lead to an improvement. Under these numerous circumstances, one is led to compare different forms of irradiation and to assess their possible biological differences. Objective description of the type of irradiation may be limited to the contents of the treatment sheet. However this does not easily express the biological quantity of the irradiation. As soon as the differences between two types of irradiation involves two parameters, they can no longer be effectively compared. The usefulness of a single figure to express the biological value of irradiation is obvious. With this aim, Cohen proposes calculation of levels of cell survival resulting from irradiation and extracts the parameters of this calculation from clinical results published in the literature. By similar analysis, Ellis has deduced the overall role of time factors which is expressed by the simple relation of the N.S.D., which has itself been at the origin of other expressions seeking to be more convenient and universal. Thus various solutions to the problem are available to radiotherapists. However a wide degree of disagreement exists between them, increasing with differences between the forms of irradiation...