The tolerance of mouse tails to necrosis after repeated irradiation with X rays.

The reduction in the "tolerance" dose for the production of tail necrosis in adult mice seven weeks after irradiation has been measured in tails which had previously received various single or multiple tolerable doses. The major findings are: A. The ability of heavily-irradiated and healed tails to tolerate about a further 90% of the first dose was found to apply between six weeks and ten months after the first irradiation. B. The tolerance dose was reduced to about 65% of the original at the third irradiation, and this dose (approximately) remained tolerable at the fourth, fifth and sixth irradiation, all delivered at six week intervals. C. Subsequent to seven weeks after the sixth dose, late effects were observed as an atrophy of the tail, mostly distal to the healed irradiated region. The occurrence was very marked (occurring in about 40% of tails by eight months) compared with that observed after only one dose (not more than 4% by eight months). Hence, if the assay time was extended to eight months after irradiation to include these late effects, the tolerance dose for the tail after the sixth dose would be reduced to about 55% of the original value.