Early and late effects of fractionated irradiation of the thorax of WAG/Rij rats.

While knowledge about early effects of radiation on normal tissues has increased considerably, due to a great number of clinical and animal investigations, very few data are available on very late effects, i.e., complications that will arise several years after treatment. One of these late complications of radiation treatment is lung fibrosis, a very slowly progressing lesion capable of completely disabling the patient several years after radiation therapy. Unconventional fractionation regimens, e.g. multiple fractions per day, are being increasingly applied in radiotherapy. When tolerance doses are to be calculated using isoeffect models like the LQ model presented by Barendsen (1982), it is important to know how late responding tissues react with changes in fractionation schemes, since the effect on early and late responding tissues might be different and late effects might be dose-limiting. The object of our studies is to determine the halftime for repair of sublethal damage (important when fractions are given at short intervals) and the a/,B ratio, using the formation of radiation fibrosis in rat lung as the endpoint.