A pilot study: Holmium emission coverage of VX2 tumors after intratumoral holmium microsphere injections using MRI and (micro)CT.

Abstract

The department of Clinical Sciences of Companion Animals of the Faculty of Veterinary Medicine and the University Medical Centre, Utrecht University are conducting experimental studies with intratumoral holmium microsphere injections to treat inoperable tumors. In the past, veterinary patients have been treated with a lot of variation in the results. This is probably due to the difference in distribution and behaviour of the particles inside the different tumour tissues. In this pilot experiment, the use of CT for holmium quantification and the effect of multiple injection sites on intratumoral holmium distribution are investigated. Seven rabbits were inoculated subcutaneously with VX2 tumor pieces on both hind limbs. Once the tumor reaches a size between 2,5 to 4 cm it received one, three or five injections with radioactive 166-Holmium poly(l-lactic acid) microspheres directly into the tumor. This was followed by imaging the pattern of distribution of the holmium microspheres using in vivo and ex vivo MRI and CT and ex vivo CT. The quality and usefulness of the different imaging techniques was compared and a dosimetric calculation was performed to calculate the % of tumor that received an adequate dose of radiation. Only three out of fourteen inoculations resulted in VX2 tumor growth on the hind limb. Just one tumor, which received five injections of holmium microspheres, resulted in a 119% dose coverage of 100Gy using dosimetry on CT images. CT and MRI did not give a 100% coverage of this tumor. In the other two tumors, the holmium microsphere injections did not result in an adequate dose coverage in any of the imaging techniques. More experiments need to be conducted in order to give any conclusions about the effect of the number of injections on resulting dose coverage. CT quantification of holmium content and distribution seems feasible, but requires further investigation and improvement.