Dose and dose-splitting effects of X-rays on lung tumour induction in mice

This paper reports lung-tumour induction 12 months after single or split doses of X-rays in C3H/He male mice. The early proliferation response of lung cells after doses which induced lung tumours was also examined after single X-irradiation. Lung-tumour incidence tended to increase with increasing dose after a single irradiation and peaked at 5 Gy. At more than 10 Gy it decreased sharply to control level. The mean tumour diameters tended to increase with doses up to 7.5 Gy and then decreased beyond 10 Gy. The lung-tumour incidence decreased with increasing intervals between two equal doses of 2.5 or 5 Gy. However, the tumour incidence after two 2.5 Gy irradiations at 1 day intervals or two 5 Gy irradiations at 6 h intervals was higher than that observed after a single dose. The labelling indices of the lung cells, determined using tritiated thymidine after a single dose. The labelling indices of the lung cells, determined using tritiated thymidine after a single irradiation, were higher than those of non-irradiated control cells. This response was delayed as the dose increased. (author)