Tumour embolization of the Vx2 rabbit head and neck cancer model with Dextran hydrogel and Holmium-poly(L-lactic acid) microspheres: A radionuclide and histological pilot study

Introduction: Intra-arterial embolization of unresectable malignant tumours with biodegradable microspheres is an effective way of selective anti-tumour therapy. Promising candidates are Dextran hydrogel (Dex) microspheres for chemo-embolization and Holmium-166 poly(L-lactic acid) (¹⁶⁶HoPLA) microspheres for radio-embolization. This study was performed to investigate the distribution of intra-arterially injected microspheres both in vivo and histologically in order to establish an optimal size of particles for embolization of head and neck tumours. Material: Twenty rabbits with V×2 auricular tumours were embolized via the caudal auricular artery with 4 different batches of microspheres: Radioactive ¹⁶⁶HoPLA microspheres sieved between 20 and 50 μm and Dextran hydrogel microspheres sieved between 20 and 100 μm (Dex20), 30 and 100 μm (Dex30) or 50 and 100 μm (Dex50). Dex20 and Dex50 microspheres were labelled with ^99mTechnetium in six cases. Methods: The average particle size of the microspheres was determined. The proportion of microspheres entrapped in the tumour was measured with a gamma camera. The distribution of microspheres around the primary tumour and spill of particles over into lungs or other organs was analysed from histological sections. Results: The mean particle diameter varied from 19 to 66 μm: ¹⁶⁶HoPLA 19 ±11 μm, Dex20 40±19 μm, Dex30 50±19 μm, Dex50 66±21 μm. The 19 μm ¹⁶⁶HoPLA particles proved inadequate for embolization as 51% spilled over into the lungs, whereas over 95% of the 40-66 μm Dex microspheres were retained within the primary tumour area. Particle density in lung tissues proved significantly lower for the Dex50 group. Stray emboli to the brain occurred in two rabbits. Conclusion: The results of this investigation show that both Dextran hydrogel and holmium-166 poly(L-lactic acid) microspheres are potential candidates for embolization of head and neck cancer. In future studies, arterio-arteriolar anastomoses which might confound treatment should be identified and occluded. Particles with a number weighted mean diameter of at least 40 μm and a volume weighted mean size up to 70 μm should be used.