[Dose of tissue tolerance, dose of tumour sterilisation in continuous and discontinuous irradiation (author's transl)].

In the radiotherapy of cancer, two conditions are necessary:-- 1 degree sterilisation of the tumor; 2 degrees safeguard of surrounding tissues. In discontinuous irradiation, sterilisation depends on the administration of a certain tumour-dose within a suitable period or number of sessions. At the rate of 5 sessions weekly, the following dose-number law may be formulated (ELLIS): DT = 2,500.N0,22. Perfect tolerance of the connective tissue is in fact obtained if the total dose is equal or less than: N.S.D..N0,24.T0,11. The nominal standard dose of ELLIS, is a limiting dose of about 1,800 rads. ORTON and ELLIS have published tables which permit one to obtain, without an index of saturation of the supporting tissue by means of a series of double entry tables corresponding each to a frequency of 1, 2, 3, 4, 5 sessions per week. Each table provides the T.D.F. factor in relation to the number of sessions and the elementary dose. The T.D.F. factor is about 100 whatever the periodicity, and whatever the subdivisions of the dose when ELLIS' equation is satisfied, i.e. when one is at the limit of tolerance. Furthermore, the T.D.F. factor of two parts, of treatment under various regimens may be added together. It is sufficient for the sum to be equal to 100 to ensure full tolerance, which eliminates all difficulty of manipulation of a fractionated formula. The conditions of obtention of continuous irradiation do not permit one to separate the doses of sterilisation and tolerance, as was the case with discontinuous irradiation but, generally speaking, both in continuous and discontinuous administration the shorter the period of treatment the more the sterilising dose exceeds the tolerance dose, whereas with very small tumours, one may obtain tumour sterilisation within the limits of tolerance of the supporting tissues. For very large or radio-resistant tumours, one may have to, in order to obtain sterilisation, exceed more or less the perfect tolerance defined by T.D.F. 100 or its equivalent using ELLIS' equation. The latter is, moreover, only an approximation : the effective tolerance, if one takes into consideration late fibrosis, is very much less than that defined by T.D.F. 100, in particular for high partial dose irradiations. The author insists, after others, on the absolute necessity of taking into consideration the time factor as defined by ELLIS' relationship recalled above. There is no sense in obtaining strict physical dose measurement if one does not also take into consideration the time factor, the biological importance of which is considerable.