Lack of apparent excess of malignant mesothelioma but increased overall malignancies of peritoneal cavity in Japanese autopsies with Thorotrast injection into blood vessels.

The carcinogenicity of thorium dioxide sol (Thorotrast), an X-ray contrast medium used in 1930-1955, in the liver and bone marrow has been established and agrees well with the effects of a high dosage of alpha radiation in the organs. Recently, however, German and Danish epidemiologic studies have shown excess mesotheliomas in the pleura and peritoneum that are unlikely to have been heavily irradiated by alpha particles. To confirm these observations, we examined the incidence of the cancer in those who underwent Thorotrast injections into blood vessels (n = 370) by using autopsy files of the Japanese Thorotrast study. Only one malignant mesothelioma of the peritoneum was registered, whereas three peritoneal or retroperitoneal sarcomas were observed. Thus, our study did not find any increment of mesothelioma in Thorotrast patients. However, when we took the pleuroperitoneal and retroperitoneal malignancies altogether, the incidence (4/370 = 1.1%) was five times more frequent than that (344/162,000 = 0.2%) in the controls (P < 0.005). Clinicopathological data of the four cases are also presented.