Thorotrast, a colloidal suspension of radioactive (232)ThO(2) that emits alpha particles, was used as a radiographic contrast agent in the 1930s-1950s. Several decades after injection, Thorotrast causes liver cancers, among which intrahepatic cholangiocarcinoma (ICC) is prominent. We investigated mutations of the RAS and the TP53 genes in archival sections of ICC induced by Thorotrast. Compared to ICC that was not associated with Thorotrast, the frequency of mutation of the KRAS gene was lower, while that of the TP53 gene was more than two times higher. The most common mutation of the TP53 gene was A-G transitions. Interestingly, TP53 mutations were also found in noncancerous areas of livers in which Thorotrast had been deposited. Furthermore, mutations tended to accumulate in tissues from more advanced tumors. These results suggest that deposited Thorotrast continuously damages DNA in liver cells in some way, resulting in A-G transitions of the TP53 gene. However, we have not been able to rule out the possibility that genetic insults occur indirectly in the proliferating cells adjacent to the necrosis rather than being a direct effect of alpha particles.