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The accumulation of 201Tl in tumor and inflammatory tissues were small. However, this nuclide showed a high concentration in viable tumor tissue, less in connective tissue (containing inflammatory tissue), and was not seen in necrotic tumor tissue regardless of the time after administration of 201Tl(I)-chloride. In inflammatory lesions, 201Tl accumulated in subcutaneous tissue infiltrated with neutrophils and macrophages, and quite large amounts of this nuclide were accumulated in subcutaneous tissue and sites where neutrophils were crowded. Most 201Tl existed in a free form in the fluid of tumor and inflammatory tissues regardless of the time after administration. A small amount of this nuclide was localized in the nuclear, mitochondrial and microsomal fractions in these tissues, and the nuclide was bound to protein in these fractions. The distribution of 201Tl(III)-chloride in tumor bearing animals was essentially the same as that of 201Tl(I)-chloride.

The biodistribution and binding substances of 95Nb and 182Ta were investigated and compared with other nuclides using tumor-bearing rats and mice. Retention values of 95Nb in tumor tissue were greater than those for 67Ga-citrate, while those for 182Ta were similar to those for 67Ga. The values for these nuclides in the liver and spleen were much smaller than those for 67Ga. However, the values for blood and some soft tissues were much greater than those for 67Ga. The concentrations of 95Nb and 182Ta were more dominant in the connective tissue (especially inflammatory tissue) than in the other categories of tumor tissue. These nuclides accumulated rapidly into the mitochondrial fraction (containing lysosome) of the liver, reaching about 50% after 48 hours, but these nuclides existed relatively uniformly in the tumor cells. The main binding substance of these nuclides in the above tissues was the acid mucopolysaccharide whose molecular mass exceeded 40,000 daltons. Radioactive niobium can be a potential tumor imaging agent. Among radioactive niobiums, 95mNb has physical characteristics (half-life 90 hours, IT decay, 234-keV energy of 100% abundance) suitable for clinical imaging study.

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