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There are few studies on the long-term sequelae of radionuclides ingested or injected into the human body. Patients exposed to radioactive Thorotrast in the 1930s through the early 1950s provide a singular opportunity, since the administration of this radiographic contrast agent resulted in continuous exposure to alpha particles throughout life at a low dose rate. We evaluated cause-specific mortality among an international cohort of 3,143 patients injected during cerebral angiography with either Thorotrast (n = 1,736) or a similar but nonradioactive agent (n = 1,407) and who survived 2 or more years. Standardized mortality ratios (SMRs) for Thorotrast and comparison patients were calculated, and relative risks (RR), adjusted for population, age and sex, were obtained by multivariate statistical modeling. Most patients were followed until death, with only 94 (5.4%) of the Thorotrast patients known to be alive at the closure of the study. All-cause mortality (n = 1,599 deaths) was significantly elevated among Thorotrast subjects [RR 1.7; 95% confidence interval (CI) 1.5-1.8]. Significantly increased relative risks were found for several categories, including cancer (RR 2.8), benign and unspecified tumors (RR 1.5), benign blood diseases (RR 7.1), and benign liver disorders (RR 6.5). Nonsignificant increases were seen for respiratory disease (RR 1.4) and other types of digestive disease (RR 1.6). The relative risk due to all causes increased steadily after angiography to reach a threefold RR at 40 or more years (P < 0.001). Excess cancer deaths were observed for each decade after Thorotrast injection, even after 50 years (SMR 8.6; P < 0.05). Increasing cumulative dose of radiation was directly associated with death due to all causes combined, cancer, respiratory disease, benign liver disease, and other types of digestive disease. Our study confirms the relationship between Thorotrast and increased mortality due to cancer, benign liver disease, and benign hematological disease, and suggests a possible relationship with respiratory disorders and other types of digestive disease. The cumulative excess risk of cancer death remained high up to 50 years after injection with >20 ml Thorotrast and approached 50%.
Few opportunities exist to evaluate the carcinogenic effects of long-term internal exposure to alpha-particle-emitting radionuclides. Patients injected with Thorotrast (thorium-232) during radiographic procedures, beginning in the 1930s, provide one such valuable opportunity. We evaluated site-specific cancer incidence and mortality among an international cohort of 3,042 patients injected during cerebral angiography with either Thorotrast (n = 1,650) or a nonradioactive agent (n = 1,392) and who survived 2 or more years. Standardized incidence ratios (SIR) for Thorotrast and comparison patients (Denmark and Sweden) were estimated and relative risks (RR), adjusted for population, age and sex, were generated with multivariate statistical modeling. For U.S. patients, comparable procedures were used to estimate standardized mortality ratios (SMR) and RR, representing the first evaluation of long-term, site-specific cancer mortality in this group. Compared with nonexposed patients, significantly increased risks in Thorotrast patients were observed for all incident cancers combined (RR = 3.4, 95% CI 2.9-4.1, n = 480, Denmark and Sweden) and for cancer mortality (RR = 4.0, 95% CI 2.5-6.7, n = 114, U.S.). Approximately 335 incident cancers were above expectation, with large excesses seen for cancers of the liver, bile ducts and gallbladder (55% or 185 excess cancers) and leukemias other than CLL (8% or 26 excess cancers). The RR of all incident cancers increased with time since angiography (P < 0.001) and was threefold at 40 or more years; significant excesses (SIR = 4.0) persisted for 50 years. Increasing cumulative dose of radiation was associated with an increasing risk of all incident cancers taken together and with cancers of the liver, gallbladder, and peritoneum and other digestive sites; similar findings were observed for U.S. cancer mortality. A marginally significant dose response was observed for the incidence of pancreas cancer (P = 0.05) but not for lung cancer. Our study confirms the relationship between Thorotrast and increased cancer incidence at sites of Thorotrast deposition and suggests a possible association with pancreas cancer. After injection with >20 ml Thorotrast, the cumulative excess risk of cancer incidence remained elevated for up to 50 years and approached 97%. Caution is needed in interpreting the excess risks observed for site-specific cancers, however, because of the potential bias associated with the selection of cohort participants, noncomparability with respect to the internal or external comparison groups, and confounding by indication. Nonetheless, the substantial risks associated with liver cancer and leukemia indicate that unique and prolonged exposure to alpha-particle-emitting Thorotrast increased carcinogenic risks.
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