Dose of tissue tolerance, dose of tumour sterilisation in continuous and discontinuous irradiation

In the radiotherapy of cancer, two conditions are necessary: sterilisation of the tumour; 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. Orton and Ellis have published tables which permit to obtain, without calculation, a time-dose fraction factor T.D.F. which represents what one might call 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 conditions of obtention of continuous irradiation do not permit to separate the doses of sterilisation and tolerance, as is 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 absolute necessity of taking into consideration the time factor as defined by Ellis relationship is underlined