The combined and separate action of neutron radiation and zirconium dioxide on the liver of rats.

To simulate the chronic alpha radiation of Thorotrast, the liver of female Wistar rats was exposed to fractionated neutron irradiation at 14-d intervals (0.2 Gy per fraction) over 2 y to a total dose of 10.0 Gy. Prior to the start of irradiation, one-half of the animals received 120 microL of non-radioactive Zirconotrast (ZrO2), which is comparable to Thorotrast with regard to all other physical and chemical properties. One year after beginning irradiation, the first liver tumor was detected. At the end of the life-span study, the incidence of irradiated animals with liver tumors was about 40%. In the animals treated additionally with ZrO2, the incidence, time of onset, and overall number of liver tumors was nearly equal, indicating that the fractionated neutron irradiation was the exclusive cause of tumor development. The lifelong-deposited ZrO2 colloid had no stimulating effect. Compared to earlier animal studies dealing with Thorotrast, the same histological types of benign and malignant liver tumors were found.