It is extremely unlikely that a particle will break free from the vessel and travel anywhere else in your body. The particles are precisely placed by the radiologist, who carefully notes the flow and pattern of the blood vessels to ensure that the particles are forced into the correct place by the pressure of the blood. Even if a particle or two were to leave the embolized area, it takes hundreds of particles to block even a tiny blood vessel, so any danger from misembolized particles is very small.