[en] ¹⁶⁶Ho-Chitosan is a complex of ¹⁶⁶Ho and N-glucosamine with 400 to 500 kD MW, which chelates metal ions and degrades slowly in vivo. In mice, ¹⁶⁶H-Chitosan administered intraperitoneally was uniformly bound to the peritoneal wall (94%), and the surface dose calculated by Monte Carlo simulation (EGS 4 code) was 81 Gy/uCi/cm². ¹⁶⁶Ho-Chitosan was administered intraperitoneally as an adjunct in the treatment of ovarian cancer with diffuse spread of malignant cells in the peritoneal surfaces including the diaphragm. 97-99% of ¹⁶⁶Ho-Chitosan was localized within the peritoneal cavity, and more than 90% of ¹⁶⁶Ho-Chitosan was attached to the peritoneal wall. Partial response were observed in 4 among 5 patients with ovarian cancer without severe toxicity. Intracavitary radiation therapy with ¹⁶⁶Ho-Chitosan in the cystic brain tumor, 5 or 8 cysts were shrunken in size with thinning of the wall, 2 out of 8 showed growth retardation. For large or multiple primary liver cancers, which were inoperable and resistent to chemotherapy. ¹⁶⁶Ho-Chitosan was used for intraarterial injection, because this solution became gel with neutral pH. In the primary liver cancer, radioactivity was distributed in the territory of selected hepatic arterial branch, and partial responses were observed in 2 cases. For the large solitary liver tumor, which was not operable due to its location, direct intratumoral injection of the radioisotope had a limited response. ¹⁸FDG PET was a useful tool to follow up those radionuclide therapy, and guide to plan the next therapy. In one case of the large single metastatic stomach cancer in the liver, ¹⁸FDG PET was done two weeks after intratumoral injection of ¹⁶⁶Ho-Chitosan, which showed cold defects matched with distribution of ¹⁶⁶Ho-Chitosan, and second injection was guided by PET image. Various methods of the administration of ¹⁶⁶Ho-Chitosan could be used for the treatment of the cancers. (author)