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Biological material itself appears with poor contrast in electron microscopy (EM), due to its composition mostly of light elements. Classical staining agents such as osmium tetroxide, uranyl acetate, and lead citrate preserve and/or stain cellular structures such as membranes, cytoplasm, and organelles well for EM. However, extracellular polymeric substances (EPS) show no or only poor contrast with these staining agents. The endothelial glycocalyx in blood vessels consists mainly of proteoglycans. It can be visualized by EM only by additional staining with heavy metal ions such as copper (Alcian blue, cupromeronic blue), ruthenium (ruthenium red), or lanthanum. Best results are achieved by combined perfusion of fixative and stain. Cationic hydrous thorium dioxide colloids (named here cThO2) trace acidic groups in EPS. We describe here the use of cThO2 to visualize the glomerular endothelial glycocalyx in the mouse kidney. cThO2 shows high electron density and binds to a continuous layer of up to a few hundred nanometers thickness on the glomerular endothelium, as well as on epithelia in other blood vessels in perfused animals. The observed staining pattern gives rise to periodic densities, with a spacing varying between 50 and 200 nm, depending on the overall layer thickness, which varies between below 50 up to 300 nm. Due to high electron density of the used cThO2 particles, the introduced method allows distinct imaging and precise fine structural analysis of the endothelial glycocalyx.

Thorotrast was used as a radiographic contrast agent in the United States from about 1930 to the mid-1950 s. Its use was discontinued when it was recognized that its radioactivity caused long-term deleterious effects. Such long-term sequelae of intravascular Thorotrast injection include, most notably, hepatic and hematologic malignancies and hepatic fibrosis. Some patients who had received Thorotrast subsequently received liver transplants. However, it was not known whether or not Thorotrast could become redistributed within the new allograft. A single report in 1994 demonstrated that radioactivity was detected by gamma-ray spectroscopy in liver allografts shortly after transplantation. No report has identified Thorotrast in the allografts of long-term transplant survivors, and the redistribution of Thorotrast into allografts has not been documented histologically or by electron microscopy. We report a case of recurrent Thorotrast into a liver allograft 10 years post-transplant. We evaluated the native liver and allograft specimens for the presence of thorium utilizing light microscopy, electron microscopy and electron X-ray microanalysis. This case report demonstrates for the first time the redistribution of Thorotrast into a long-surviving liver allograft using histologic, electron microscopic and X-ray microanalysis techniques.

Purpose: The aim of this study was to assess late effects of long-term exposure to alpha irradiation caused by paravascular Thorotrast deposits., Subjects and Methods: 899 patients, who had received the radioactive contrast medium Thorotrast for angiography in the 1930s and 1940s, and 662 controls were followed-up since 1968 every two years by standardized clinical and laboratory examinations. Initially, X-ray plain films of the thorax, upper abdomen and the former injection site were performed. In selected patients the sites of paravascular Thorotrast deposits were evaluated by ultrasonography, CT and MRI., Results: Paravascular Thorotrast deposits were detected in 245 patients. Clinical symptoms related to deposits appeared 10 to 30 years after Thorotrast administration. The severity of symptoms depended on the location and extension of granulomas and were mainly caused by fibrosis, nerve paralysis and vascular changes. Four malignant tumors adjacent to granulomas were observed (one soft tissue sarcoma in the groin, two squamous cell carcinomas of the parotid gland and one lymphoepithelial carcinoma of the nasopharynx). MRI including MRA allowed an accurate determination of tissue damage, whereas the utility of US and CT was restricted due to strong sound attenuation and streak artefacts caused by the high X-ray absorption of Thorotrast., Discussion and Conclusion: Locoregional late effects of paravascular Thorotrast deposits mainly comprise radiation induced, fibrotic tissue destruction. The incidence of malignant tumors, in particular sarcomas, adjacent to deposits, however, is much lower than initially expected.

Simplified and reliable methods for identifying thorotrast are demonstrated. On gamma-ray spectrometry, as little as 0.1 g of liver tissue gives gamma-ray spectrum characteristics to the thorium decay series, despite a low net count. When conventional film for photography is kept in contact with a paraffin section of the liver for 4 months, it develops silver grains exactly corresponding to the area of granular deposition. By combining this method with clinical and pathological findings, a diagnosis of thorotrastosis can be established without the need for special instruments or materials.

Vinyl chloride is a potent hepatocarcinogen which reacts with DNA to generate etheno bases. In order to determine whether mutational patterns in target genes in vivo are characteristic of vinyl chloride and could be explained by the mutagenic properties of the etheno bases, human and rat liver tumours associated with exposure to vinyl chloride were analysed for point mutations in the ras and p53 genes. In this paper, we review these data and report our latest results on animal tumours. Two alterations were found which could be attributed to a direct effect of vinyl chloride: a GC-->AT transition which leads to a GGC-->GAC mutation at codon 13 of the Ki-ras gene in human liver angiosarcomas, and lesions at AT base pairs, mostly AT-->TA transversions, which lead to mutations in the p53 gene in human and rat angiosarcomas and to a CAA-->CTA mutation at codon 61 of the Ha-ras gene in rat hepatocellular carcinomas.

Thorotrast, a 25% colloidal suspension of 232ThO2, was formerly used as a radiographic contrast medium. Although epidemiological studies have shown that alpha particles emitted from 232Th and its decay products incorporated in the bone marrow cause leukemia, the use of these data for alpha particle induced leukemogenesis risk estimation has been criticized mainly for inhomogeneity of Thorotrast distribution. Four monkeys were injected with Thorotrast to investigate the degree of inhomogeneity in the thorium content of different bone marrow sites and the cellular localization of Thorotrast. Two were injected via an artery and two via a vein and sacrificed either at 1 wk or 3 to 4 y after injection. Microscopic, solid state autoradiography and back scatter electron imaging methods were applied to several bone sites to determine the degree of inhomogeneity. Quantification was performed using x-ray fluorescence for trabecular bone and bone marrow and neutron activation analysis for compact bones. At 1 wk Thorotrast was found to be distributed evenly in the red marrow; by 3 and 4 y conglomerates were seen which were restricted to macrophages. The monkey was found to be a good model for humans. The choice of injection route did not noticeably affect the Thorotrast distribution in bones of the skeletal system. Considering the even distribution of Thorotrast within the red bone marrow at early times after its injection, the inevitable diffusion of thorium progeny from the particles, the mobility of bone marrow macrophages, and the well established correction factor of self-absorption within conglomerates, these results suggest that data derived from Thorotrast patients are useful for risk estimation of alpha particle induced leukemia.

At a metallurgical facility using thorium dioxide feed materials, a number of conventional direct- and indirect-bioassay methods were potentially suitable monitoring methodologies when industry-standard dose assessment methodologies were based upon ICRP recommendations issued in 1959. When the ICRP recommended a different dose assessment methodology in 1977, the intake limits for thorium dropped significantly, and conventional bioassay methods for routine monitoring were rendered ineffective. While determining the technical basis for its air monitoring program, the facility noted that the radioactive materials used as feed to the process were highly insoluble. Therefore, their solubility in simulated lung fluids was determined. The results of this determination indicated that the facility's materials, if inhaled, dissolve in the lung at a half-time that is significantly longer than that contained in international consensus standards for Y-class compounds. They also indicated that the secondary dose limits, if adjusted to eliminate the dose contribution from translocated radionuclides, should be higher, and that a derived air concentration based upon more recent international recommendations is more appropriate for use in the facility's routine monitoring program.

To investigate whether there is a permanent translocation of Thorotrast in the body, the liver of male Lewis rats was removed 4 weeks after injection of Thorotrast (300 microliters) and replaced by a donor liver. In half of the animals the spleen was removed as well. Measuring the 232Th content in the donor liver at different times after implantation demonstrated a permanent transport of 232Th into the surrogate organ. After 231 days a 232Th depot of about 1.1 mg was found, representing about 3% of the total body burden. The additional removal of the spleen resulted in a significantly lower transport of 232Th into the implanted liver. Histological examinations of the donated livers revealed increasing local concentration of Thorotrast granules, leading to the development of conglomerates. A comparable translocation of Thorotrast was verified in two humans who required liver transplantation more than 40 years after Thorotrast injection.

To investigate the possible influence of anticonvulsant treatment on cancer risk, a nested case-control study of 104 lung cancers, 18 bladder cancers, and 322 cancer-free controls was conducted. The background for the study was previous observations among 8004 epileptics in Denmark with a significantly high risk for lung cancer and a significantly low risk for bladder cancer. Cigarette smoking appears to explain the lung cancer excess but not the low risk for bladder cancer, another tobacco-related disease. Information was abstracted on 94 and 95% of the cases and controls, respectively. Lung cancer was not associated with any anticonvulsant drug, but bladder cancer was inversely related to use of phenobarbital (PB). The apparent protective effect of PB was further evaluated in a study of rats given 4-aminobiphenyl (ABP), a bladder carcinogen. The levels of 4-aminobiphenyl adducts in hemoglobin and in bladder and liver DNA were significantly lower in rats given PB prior to 4-aminobiphenyl, compared to controls. These studies suggest that PB may induce drug-metabolizing enzymes of the liver that deactivate bladder carcinogens found in cigarette smoke and provide clues to the role of activation and detoxification of carcinogens in humans.

It is well established that injected Thorotrast is deposited in the liver, spleen, bone marrow, and lymph nodes, but accumulations in organs with lower macrophage activity have previously been given little attention. In this work, neutron activation analysis has been used to investigate concentrations of thorium in autopsy samples taken at sites of major and minor deposition in 24 Thorotrast patients. In the latter category, the highest values were found in the testis [40 x 10(-6) g/g(wet)], followed by those in the adrenal gland, gallbladder, lung, and pancreas. The resulting alpha-particle dose rates (mGy/year) are tentatively estimated to be 8.5 to the testis, 5.5 to the gallbladder, and 5.3 to the lung. These results may be relevant to the residual excess mortality among Thorotrast patients after diseases of the principal organs of deposition have been excluded; they also support previous indications that thorium deposited in pulmonary tissues is responsible for an important component of the total dose to the lung. In another context, our data may bear on the connection, postulated elsewhere, between exposure to alpha-particle emitters and elevated incidence of leukemia in the children of workers engaged in the reprocessing of nuclear fuel.

Finely reticulated or granulated shadows of liver and spleen, as well as metal-dense shadows of abdominal lymph nodes are characteristic roentgen findings in thorotrastosis. Thorium dioxide (Thorotrast) was used as a contrast medium from 1930-1950. It yielded excellent contrast and was well tolerated, but due to its radioactivity and its long biological half-life it caused manifold late complications. The typical roentgen findings in thorotrastosis are illustrated by two cases; the importance of thorotrastosis is discussed on the basis of relevant literature.

Guinea pig metaphyseal bone was exposed to horse spleen ferritin in vitro and to colloidal thorium dioxide in vivo. The cellular uptake and intracellular accumulation of these marker particles were studied ultrastructurally. In vitro, the ferritin molecules were found to spread evely throughout the tissue. After 1-2 hours ferritin was mainly found in plasma membrane invaginations and in endocytic vesicles of varying size. At 4-6 hours a successive accumulation of the marker in secondary lysosomes could be observed. In addition to ferritin, the lysosomes and the large endocytic vesicles often contained other inclusions. In vivo, the pattern of intracellular accumulation of the marker particles was identical to that in vitro. Moreover, the presence within the cells of similar amounts of thorium dioxide after 1 and 4 days suggested that these indigestible molecules are stored intracellularly for a considerable time. In accordance therewith there were no definite signs of extrusion of labeled bodies or secretion of the exogenous marker by exocytosis. Ferritin and thorium dioxide were taken up by all cell types in the metaphysis. Both in vitro and in vivo perivascular cells type B ingested large amounts of marker particles, whereas chondroclasts, endothelial cells. perivascular cells type A and osteoblasts showed a more restricted endocytizing ability. On the basis of these observations, the functional significance of different cell types in the resorption of the epiphyseal cartilage and the formation of bone is discussed.

Takophrya infusionum like all other Suctoria lacks an oral cavity. Its feeding apparatus consists of tentacles, long narrow tubes through which the contents of the living prey are ingested. For normal growth, reproduction, and longevity of clones, Tokophrya needs supplements deriving from the medium in addition to living prey. Since Tokophrya lacks a mouth, these supplements can reach the cytoplasm only through the complex structure of the cortex, which is composed of a three-membraned pellicle and a dense epiplasm. In addition, external to the cortex, an extraneous coat covers the whole organism. Only the outer pellicular plasma membrane is continuous; the other two and the epiplasm are interrupted by the outer plasma membrane which invaginates at intervals forming the so-called pits. The invaginated plasma membrane dips down into the cytoplasm where it extends to form a saccule. Experiments with cationized ferritin and Thorotrast provide evidence that internalization of these macromolecules takes place through the pits by pinocytosis. The membrane of the saccules of the pits forms invaginations which pinch off giving rise to small, flattened vesicles containing the tracers. The tracers were never found free in the cytoplasm but exclusively in the flat vesicles. These vesicles are thus the vehicles transporting macromolecules from the medium to the cytoplasm. The saccules of the pits are the natural loci of pinocytosis and together with the flattened vesicles perform an important function in Suctoria, supplying the organisms with macromolecules from the medium.

The ultrastructual localization of thorium dioxide was examined in liver biopsy specimens from two patients injected more than 30 years before with thorotrast. An energy dispersive X-ray microanalysis spectrometer (Kevex 5100) was used to identify thorium in the liver tissue. By electron microscopy, most of the thorium particles were found in the cells of the reticuloendothelial system, such as macrophages of the portal triad and Kupffer cells in the hepatic sinusoid. Thorium dioxide particles were mainly located within the phagosomes, but larger aggregates existed in the cytoplasma with no visible limiting membrane. In addition, the deposition of thorium granules in a few hepatocytes was also confirmed by electron microscopy. This fact indicates that two main pathways of elimination are still functioning in the late period, but the hepatocytic pathway appears to be less effective than the reticuloendothelial system.

Small tracers in the circulation enter the rat mesangium rapidly and in large amounts that indicate a sizable plasma flow into the mesangium. Entrance is effected through mesangial fenestrations with a mean width in scanning electron microscopy of 376 A, a size similar to fenestrations in peripheral glomerular capillary walls. This is considerably smaller than the mean size of 678 A found with transmission electron microscopy, but the difference is probably due largely to the anionic surface coat on endothelial cells. Measurements of asymmetric thorium dioxide particles show that smaller ones with a mean length of 315 A enter the mesangium preferentially and that larger particles with a mean length of 405 A are partially restricted, supporting the idea that the measured width in scanning electron microscopy is close to the actual width in vivo. Fluid flow into the mesangium requires fluid flow out. The appearance time and accumulation of tracers suggest the following exit paths of flow from the mesangium: through the overlying epithelium into the urinary space contributing to glomerular filtration and concentrating large tracers beneath the basement membrane in the paramesangial region, into the efferent glomerular capillaries after tracers have been filtered out by the fibrillar matrix, and through the hilus into the juxtaglomerular apparatus (quantitatively small).

A carcinoma of the renal pelvis was diagnosed in a female patient 42 years after a retrograde Thorotrast pyelogram. The patient died as a result of the carcinoma at the age of 60. The literature is reviewed. As a result of this case, the need is stressed for early recognition of Thorotrast depositis in the kidney. In this case it was demonstrated by autoradiography. Appropriate treatment is discussed.

The ability of the mouse mast cell to phagocytize colloidal thorium dioxide, Thorotrast, was investigated employing the mouse connective tissue air pouch. The connective tissue mast cell of mouse was found to have limited capability to ingest the particulate Thorotrast in comparison to the rat peritoneal mast cell which ingested this material readily. Fibroblasts and macrophages in the connective tissue removed injected Thorotrast very rapidly in contrast to mast cells that demonstrated limited phagocytic capabilities. The tissue mast cell of the mouse, therefore, should not be considered a part of the reticuloendothelial system.

A patient is presented in whom multiple lienal circular foci are seen 25 years after application of a thorium-containing contrast medium for vascular visualization, over and above the well-known characteristic ThO2 deposits in the liver and spleen. The article discusses the problem whether these circular foci which meet the angiographic criteria of an angioblastic sarcoma, should be considered as a thiorium-induced neoplasia.

As a presupposition for estimating the mean tissue dose from intravascularly injected Thorotrast results of investigations on tissue distribution and steady state activity ratios of 232Th and daughters in Thorotrast patients were compiled and are presented as "best estimates". Special emphasis has been given to the non-uniformity of Thorotrast distribution on the organ and cellular level on the basis of results from animal experiments. Moreover, the variation widths of the mean tissue doses were calculated from the individual standard errors of the mean Thorotrast tissue distribution and activity ratios. According to the results of Thorotrast tissue distribution analyses about 97% of intravascularly injected colloidal ThO2 are retained by the organs of the reticulo-endothelial-system (RES) of the average Thorotrast patient (liver: 59%; spleen: 29%; bone marrow: 9%). Only 0.7 and 0.1% are distributed within the lungs and the kidneys, respectively. The fractional retention of 232Th in the marrow-free skeleton proved to be 2% on the average. Considering in addition the results on the steady state activity ratios between 232Th and its daughters and self-absorption of alpha-energy in Thorotrast agglomerates the mean annual tissue doses to the liver, spleen, red bone marrow, lungs (respiratory zone), and cells on bone surface, e.g., from 30 ml intravascularly injected Thorotrast are about 30 (10-70), 80 (30-200), 10 (4-27), 4.5 (1.8-11.3), and 15 (6-38) rad. The variation widths of the mean tissue doses given in brackets are based upon an average individual standard error of the mean Thorotrast tissue distribution and activity ratios of 150%. The data on mean tissue doses, however, do not include variations of the dose due to macroscopic inhomogeneities of Thorotrast distribution on the organ level, which in the liver may go up to a factor of 50. Contrary to the mean tissue dose the local annual dose, i.e., the dose to cells adjacent to the surface of 0.1-50 micron Thorotrast aggregates is between 40 and 40,000 rad.

Thorotrast is a X-ray contrast medium which has been used in the years 1930-1950. It is hardly ever eliminated from the body and is stored in the organs of the reticulohistiocytic system (RHS). Since Thorotrast contains all radionuclides of the thorium decay chain, the effect of internal irradiation by these radionuclides can be investigated in the so called "Thorotrast patients". It is the purpose of this study to determine the radiation dose absorbed by the lymphocytes of the peripheral blood of these persons, and to correlate it with their chromosome aberration rate. Part I of the paper describes the physical aspects of dose evaluation. The chromosome aberration rates of Thorotrast patients were subjected to a repeated statistical test. They show a logarithmic normal distribution at a mean value of about seven dicentric chromosomes per 100 cells. The corresponding dose has to be determined. The method for evaluating size and distribution of Thorotrast conglomerates in the examined tissue is demonstrated, and the function and resolution power of the applied "texture analysing system" (TAS) is described. Size-distributions of Thorotrast conglomerates of lymph nodes from the neck, the lung and from the lower aorta (bifurcatio) are given. In the lymphatic system more than 90% of the absorbed dose in tissue is due to alpha-particles. The computation method of calculating the self-absorption of alpha-particles in the conglomerates and of calculating the annual doses in the lymph nodes is described. The values lie between 135 and 1200 rad/year. The contribution to irradiation from outside the lymphatic system can be neglected. The method of dose determination is discussed and compared with microdosimetric methods. With these methods, the dose of a lymphocyte caused by one hit of an alpha-particle lies between 80 and 920 rad. An energy dissipation does not take place and the total energy emitted acts on the lymphocytes thus inducing chromosome aberrations.

H-2b lymphocytes were sensitized against H-2d alloantigens by a lymphocyte culture reaction (MLR) and incubated with H-2d mastocytoma cells. Thirty minutes before incubation with the mastocytoma cells Thorotrast was added to the lymphocyte culture medium. The interaction between lymphoid cells and mastocytoma cells was stopped by fixation with glutaraldehyde. The areal densities of the cytoplasmic vacuoles were examined by electron microscopy. Two populations of lymphocytes were observed, small lymphocytes with heterochromatic nuclei and larger lymphocytes (lymphoblasts) with euchromatic nuclei. Only the lymphoblasts showed changes following interaction with target cells. The areal density of Thorotrast containing vacuoles in sensitized lymphoblasts increased during the three hour observation period, but the percentage of vacuoles containing Thorotrast in the total area of cytoplasmic vacuoles decreased in the first thirty minutes of interaction with target cells. This observation may indicate greater formation of vacuoles from the Golgi apparatus compared with the formation of vacuoles from the cytoplasmic membrane at the onset of interaction with target cells. In the multivesicular bodies (MVB) the Thorotrast was first observed inside the small vesicles and later on it was also observed in the matrix of the MVB. After three hours of interaction Thorotrast was observed laying "free" in the matrix of phagosomes along with myelin figures.

Fusion of lysosomes with phagosomes containing Ehrlichia risticii, an obligate intracellular parasite, was evaluated in P388D1 murine macrophagelike cells. Lysosomes in cells ranging in infectivity from 30 to 70% were labeled cytochemically with acid phosphatase or via endocytosis of thorium dioxide or cationized ferritin to document phagosome-lysosome (P-L) fusion in untreated cells and cells treated with oxytetracycline. Regardless of the marker used, P-L fusion was generally not observed in E. risticii-containing vacuoles in untreated cells, while significantly greater P-L fusion with ehrlichia-containing vacuoles was observed after oxytetracycline treatment. When latex beads were introduced into uninfected cell cultures, P-L fusion was observed with vacuoles containing latex. Fusion of lysosomes with latex-containing vacuoles in cells was significantly greater than fusion of lysosomes with ehrlichia-containing vacuoles in the same infected cells. These findings indicate that E. risticii is able to inhibit P-L fusion, whereas oxytetracycline deprives organisms of this ability.

Unusual inclusions, which occurred in the reticuloendothelial cells intimately associated with fresh amyloid deposits, were analyzed by electron microscopy. The inclusions were located in the areas rich in the primary lysosome type of dense bodies and the cytoplasmic invaginations containing well-oriented amyloid fibrils. They were single-membrane-bounded, measured 0.3 to 0.8 mu in width and 0.5 to several microns in length, and showed considerable variation in the electron density of their contents. The latter consisted of two different ultrastructural elements: fibrillar profiles and a homogeneous or finely granular electron-dense substance. The fibrillar profiles were virtually identical in ulstrastructure to the amyloid fibrils and were well-oriented parallel to the long axis of the inclusion. The homogeneous or finely granular electron-dense substance appeared to be comparable to that composing the dense body matrix. The inclusions were usually acid phosphatase positive, but did not take up intravenously injected Thorotrast particles. These data led us to conclude that these inclusions were transitional forms from the usual dense bodies to the deep cytoplasmic invaginations containing well-oriented amyloid fibrils (which are accepted by most investigators as the sites of amyloid formation) and thus constitute direct evidence for the involvement of lysosomes in amyloid fibril formation.

Sensory ganglia from 9-day chick embryos were grown on collagen coated coverslips for36 h in the presence of nerve growth factor, producing a profuse neuritic outgrowth. The cultures were then incubated for varying periods in a colloidal suspension of thorium dioxide, and the pinocytotic uptake of this marker was followed by electron microscopy. Following brief exposures (3 min), most of the labelled organelles consisted of smooth surfaced vesicles and vacuoles; with longer exposures, the bulk of the marker accumulated first in cup-shaped pre-multivesticular bodies and ultimately in multivesicular bodies. The marker was also taken up into coated vesicles, dense-cored and electron lucent tubules,dense-cored vesicles and dense bodies of the multi-layered myelin body configuration. In addition, evidence suggestive of exocytosis was also obtained; views of apparent fusion of labelled multivesicular bodies with the plasmalemma involving extrusion of vesiclesand marker particles into the extracellular space were regularly encountered following long exposures.

As a prerequisite for quantifying the non-radiation effect of Thorotrast, nonradioactive and radioactive aquasols with identical physicochemical properties and with biophysical behavior comparable to that of Thorotrast were developed and produced for a second long-term animal experiment. Comparative investigations with hafnium and zirconium (zirconotrast) dioxide aquasols showed the latter to be most appropriate considering the size of both the dispersoids and the aggregates in the liver tissue. The average particle diameters of ThO2, ZrO2 and HfO2 proved to be values of 9.3 nm, 15 nm and 45 nm, respectively. The size of the aggregates shows a slight dependence on the applied amount but no dependence on the duration of body burden of the colloid. The aggregate diameters in the 600 microliters group turned out to be 9.6 microns, 14.4 microns and 5.3 microns, respectively. Radioactive zirconotrast was prepared by radiochemical incorporation of 230Th and 228Th at dose rates which produce accumulated doses in the liver of rats equivalent to those of commercial 230Th enriched Thorotrast after 1.5 yr. Five different colloids were prepared with alpha-energy emission rates increased by factors of 1, 2.5, 5, 10 and 25 compared to Thorotrast and then injected into rats.

The biological effect of alpha-radiation, which has been examined in this investigation, is the occurrence of chromosome aberrations in lymphocytes from the peripheral blood of Thorotrast patients. To set up a dose-effect relationship, it is necessary to estimate the absorbed dose of a single lymphocyte found in a blood sample. Then it would be possible to use chromosome aberrations of lymphocytes from peripheral blood as an indicator for estimating the effective radiation dose. The physical aspects of this investigation have been described in part I of this paper. In part II, biomedical considerations are described and discussed. The irradiation of the lymphocytes takes place in the whole lymphatic system, but the lymph node could have been used as a model for the lymphatic system. Based on the evaluation of size and distribution of Thorotrast conglomerates, the average annual dose in different structures of the lymph node has been determined. The Thorotrast content in lymph nodes depends on the region of the body from where the lymph nodes originate. It has been tried to compile the distribution of the lymph nodes in the different regions of the body. Annual doses have been calculated in these different regions in order to find a representative value for the whole body. The values describing the kinetics of the lymphocytes in the body have been used to convert the annual dose in lymph nodes into the dose of a lymphocyte found in a sample of peripheral blood. It is pointed out that the calculated dose of the lymphocytes, being 250 rad, is an estimation of an upper value. The lymphocytes are "in vivo" irradiated by the Thorotrast, but most of the calibrations used for biological dosimetry are referred to as "in vitro" experiments. The discrepancy between "in vivo" and "in vitro" results are discussed in the findings of this study and in other results.