Abstract: A method of distributing microparticles is provided, the method comprising: providing a plurality of microparticles at an insertion site in a medium; applying ultrasound to the insertion site that generates gas bubbles by cavitation at cavitation nuclei located at the insertion site and drives movement of the gas bubbles such that the gas bubbles drive movement of the microparticles into a desired spatial distribution in the tumour. The method may be a method of treating a tumour, and the microparticles may comprise a radioisotope for treating the tumour. Microparticles for use in the treatment of a tumour by the method are also disclosed.