INTRODUCTION: Cancer is a major cause of death in cats and dogs. Tumours are often discovered by owner or veterinarian when it is too late for surgical removal. In that case a promising alternative treatment option could be internal radiation therapy by using radioactive holmium microspheres. There are two ways in which these radioactive microspheres can be administered: by interstitial microbrachytherapy (intratumoral injection) or intra-arterial microbrachytherapy (transcatheter injection). This study is divided into two substudies. SUBSTUDY 1: Holmium microspheres (HoPLLA-MS) are injected into the hepatic arteries of human livers with tumours and without tumours to examine the distribution of microspheres in these livers. This distribution is visualized by different imaging modalities (CT, SPECT, MR imaging and phosphor imaging). SUBSTUDY 2: Holmium microspheres (HoAcAc-MS and HoPLLA-MS) are used for intratumoral injection in veterinary patients with different kinds of inoperable tumours. Three patients are treated; a Rottweiler with osteosarcoma, a Jack Russell Terrier with pituitary macro adenoma and a Persian cat with malignant melanoma. DISCUSSION: The distribution of microspheres in the liver with tumours is very different from that in the liver without tumours. The microspheres appear to be located around the tumours. The study design seems to be largely useful for future studies on this matter. Interstitial microbrachytherapy by using holmium microspheres appears to be applicable in animals with pituitary macroadenomas. CONCLUSION: The results of this pilot study on the distribution of holmium microspheres in human livers and the use of holmium microspheres in veterinary patients are promising but more research should be done before any conclusions can be drawn
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