Abstract: Devices for monitoring cancerous cells in blood of a patient that are preloaded with a marker and a magnetic agent and removing them are provided. The device comprises a tray onto which blood withdrawn from the patient flows before returned to the patient; a magnet placed at a distance from the tray so that the blood on the tray is within a magnetic field of the magnet and the cancerous cells that are preloaded with the magnetic agent are forced to move from the blood on the tray towards the magnet. A detector configured to detect the marker in the cancerous cells in the blood on the tray is provided as well and a filter that is placed between the tray and the magnet so as to capture the cancerous cells that are forced to move towards the magnet and remove them from the blood of the patient.