Allelotypic Characteristics of Thorotrast-Induced Intrahepatic Cholangiocarcinoma: Comparison to Liver Cancers not Associated with Thorotrast

The uneven irradiation of a target cell and its dynamic movement can mathematically explain incubation period for the induction of cancer by internally deposited radionuclides

Irradiation from internally deposited radionuclides induces malignant tumors. Ingested radionuclides accumulate in specific organs, which are irradiated over a lifelong period. Our aim is to elucidate why the development of malignant tumors requires long-term internal exposure, on the order of decades, despite the fact that irradiation is continuous over this period. Three major factors are considered to be responsible for the long incubation time in carcinogenesis caused by internally deposited alpha-emitters: uneven distribution of radionuclides, limited range of irradiation, and dynamic movement of tumor precursor cells. We hypothesized that target cells susceptible to malignant transformation may undergo one event by alpha particles and may then migrate outside of the range of alpha particles, thereby avoiding immediate induction of successive additional events that would lead to cell death or neoplastic changes. Based on this hypothesis, we further proposed a mathematical model to predict the relationship between dose rate and incubation period of tumors induced by internally deposited alpha-emitters. The function was non-linear and included terms of both direct and indirect radiation effects. It well fitted both human Th-ICC cases and rat Pu-induced lung cancer, suggesting that indirect radiation effects are independent from dose rate. The significance of parameters of the model is discussed.

Molecular evidence for the neoplastic potential of hepatic Von-Meyenburg complexes

Von-Meyenburg complexes (VMCs) have been shown to progress to cholangiocarcinoma (CC) in some cases. Histologic examination in such cases reveals a gradual transition of VMCs to intermediate lesions and finally to CC. The goal of this study was to determine if this histologic progression was also accompanied by sequential genetic alterations. Two cases that showed many VMCs with a transition to cholangiocarcinoma through intermediate lesions were analyzed. Multiple VMCs (away from the tumor), intermediate lesions and areas of frank CC were microdissected under stereoscopic guidance and were analyzed for allelic imbalance [loss of heterozygosity (LOH)] using a panel of 20 polymorphic microsatellite markers by polymerase chain reaction/electrophoresis. The 2 cases of CCs revealed LOH at 5 and 7 different genomic loci specific for each patient, respectively. Coexisting VMCs also exhibited LOH ranging from 0 to 3 loci in each case. Intermediate lesions showed LOH at a single locus in case 1, whereas the assay could not be performed in case 2 due to inadequate DNA yield. In case 2, the earliest acquired mutations were present in the VMCs supporting a causal relationship for neoplastic progression. Discordant LOH mutations were also present in the VMCs and the adenomatous lesions providing support that LOH detected at these sites did not passively migrate or reflect contamination. The results of the present study show that the histologic progression observed through these stages is also accompanied by LOH at loci harboring key oncogenes. The findings also support that VMCs are preneoplastic lesions that could progress to CC with time.

Histological type of Thorotrast-induced liver tumors associated with the translocation of deposited radionuclides

Exposure to internally deposited radionuclides is known to induce malignant tumors of various histological types. Thorotrast, a colloidal suspension of radioactive Thorium dioxide ((232)ThO(2)) that emits alpha-particles, was used as a radiographic contrast during World War II. Thorotrast is known to induce liver tumors, particularly intrahepatic cholangiocarcinoma (ICC) and angiosarcoma (AS), decades after injection. Therefore, patients injected with Thorotrast comprise a suitable study group to understand biological effects of internal ionizing radiation injury. Autoradiography and X-ray fluorescence spectrometry (XRF) were carried out on non-tumorous liver sections from Thorotrast-induced ICC (T-ICC) and Thorotrast-induced AS (T-AS). Autoradiography revealed that the slope of the regression line of the number of alpha tracks for the amount of deposited Thorium ((232)Th) was higher in non-tumorous parts of the liver with T-ICC than those with T-AS. XRF showed that the intensity ratio of Radium (Ra) to Thorium (Th) in non-tumorous liver tissue with T-ICC was significantly higher than that with T-AS. Furthermore, the mean (228)Ra/(232)Th radioactivity ratio at the time of death calculated was also significantly higher in T-ICC cases than in T-AS cases. These suggest that the metabolic behavior of radionuclides such as relocation and excretion, as well as the content of deposited radionuclides, is a major factor in determining the histological type of Thorotrast-induced liver tumors.