[Tumor inactivation and radiobiology]

In this paper, the inactivation of tumors and the tolerance of irradiated surrounding normal tissues is considered under radiobiological aspects. Surviving euoxic cells may be calculated by means of a simple exponential function being characterized by a reduction constant D0. This constant decreases with increasing single doses. Another exponential function allows the calculation of the probability of tumor destruction from the surviving fraction of tumor cells after irradiation. Examples show the probability curves steeply rising with the total dose, in accordance with former observations of Holthusen (S-curves). From the amount of total dose and the steepnees of the rise the values of radiation sensitivity of the particular cells may be evaluated. If there is a fraction of anoxic tumor cells, it is probable that after every irradiation a part of them is transformed into euoxic cells. In such a case, the recovery curves, depending on the degree of reoxygenation, will shift to larger values of the total doses. A good reoxygenation, therefore, is highly important for the success of the irradiation, it can be realized by appropriate irradiation programs. The regeneration (repopulation) of normal cells is of fundamental importance for the restitution of normal tissues following radiation injury. It is generally assumed that the tolerance dose is reached when the number of surviving normal cells arrives at a lower limit. Cohen's measurements at the skin of patients treated with 200 keV X-rays have been used for the calculation of tolerance doses for Co-60 gamma-rays and fast electrons. The tolerance doses depend on the amount of the single doses and result in calculated values of over 10000 rd with electron irradiation of 500 rd per week. These calculations are confirmed by the findings of Schumacher with electron irradiation.