Understanding how size and shape can affect the biodistribution of intravascularly injected particles is of fundamental importance both for the rational design of delivery systems and from a standardization and regulatory view point. In this work, uncoated silica spherical beads, with a diameter ranging from 700 nm to 3 microm, and uncoated non-spherical silicon-based particles, with quasi-hemispherical, cylindrical and discoidal shapes, have been injected into tumor bearing mice. The number of particles accumulating in the major organs and within the tumor mass has been measured through elemental silicon (Si) analysis. For the spherical beads, it has been found that the number of particles accumulating in the non-RES organs reduces monotonically as the diameter d increases, suggesting the use of smaller particles to provide a more uniform tissue distribution. However, discoidal particles have been observed to accumulate more than others in most of the organs but the liver, where cylindrical particles are deposited at a larger extent. These preliminary results support the notion of using sub-micrometer discoidal particles as intravascular carriers to maximize accumulation in the target organ whilst reducing sequestration by the liver.