Response of rat brain tissue for the extraction of 125I antipyrine after single and fractionated roentgen ray doses. A comparison of fractionation models applied to the central nervous system.

Changes in the cerebral blood flow in rats were studied 9 months after irradiation with single and fractionated doses of 250 kV roentgen rays. The single doses used were 11.9, 14.5, 17.0, 19.6 and 22.1 Gy. Corresponding fractionated doses (FD) were calculated by the formula: FD = single dose X N0.42, where N is the number of fractions. The fractionated doses were given as 4, 7 and 10 dose fractions in the same overall treatment time of 3 weeks. The blood flow changes were estimated by the 125I antipyrine extraction technique. In the non-irradiated control group the extraction in the brain was 0.93 per cent of the injected dose of 125I antipyrine per g of tissue. In the irradiated rats the corresponding extraction was on average 20.4 per cent higher than that in the control group. The extraction was significantly increased in 7 of the 20 irradiated groups of animals. The fractionation model used in these experimental studies is compared with other published fractionation models for radiation tolerance of the rat and human brain and spinal cord.