A system for operating an extracorporeal blood treatment device, for removal of circulating tumor cells (CTCs), from blood of a mammalian patient, said system comprising: - a dialysis machine; - a dialysate container for storing a dialysate to be used in a blood treatment process; - the blood treatment device attached to said dialysis machine and said dialysate container via respective pipes (430, 431, 432, 433) for circulating the blood of a patient and a dialysate through the blood treatment device; - pipes (434, 435) for withdrawing and returning a patient's blood to/from said dialysis machine; - wherein said dialysis machine comprises a control means for controlling a linear flow velocity of the blood within the hollow fibers of the blood treatment device within a range of 1.8×10 -5 m/s to 7×10 -5 m/s along a substantial length of the hollow fibers, thereby maintaining normal low shear conditions of blood in the hollow fibers to allow pathogenic cells to form clusters in the flowing blood and/or on the membrane surface of the hollow fibres once a CTC is bound by the binding agent, wherein the CTC serves for binding further CTCs, and said blood-treatment device comprising: - a house having a blood inlet, blood outlet, a dialysate inlet and a dialysate outlet, - a filter unit arranged within the house, said filter unit being formed by a plurality of hollow fibers, each hollow fiber having a predetermined length and a wall in the form of a semi-permeable membrane with a pore size of at most 1 micron; - wherein said plurality of hollow fibers provide a fluid communication between the blood inlet and the blood outlet, and the space outside the hollow fibers provides a separate fluid communication between the dialysate inlet and the dialysate outlet, - wherein the inner diameter of the hollow fibers is in the range of 100 to 400 microns, and - wherein at least a part of the inner surfaces of the fiber membrane adapted to contact the blood is coated with a binding agent for binding the CTCs flowing in the blood stream. - a dialysis machine; - a dialysate container for storing a dialysate to be used in a blood treatment process; - the blood treatment device attached to said dialysis machine and said dialysate container via respective pipes (430, 431, 432, 433) for circulating the blood of a patient and a dialysate through the blood treatment device; - pipes (434, 435) for withdrawing and returning a patient's blood to/from said dialysis machine; - wherein said dialysis machine comprises a control means for controlling a linear flow velocity of the blood within the hollow fibers of the blood treatment device within a range of 1.8×10 -5 m/s to 7×10 -5 m/s along a substantial length of the hollow fibers, thereby maintaining normal low shear conditions of blood in the hollow fibers to allow pathogenic cells to form clusters in the flowing blood and/or on the membrane surface of the hollow fibres once a CTC is bound by the binding agent, wherein the CTC serves for binding further CTCs, and said blood-treatment device comprising: - a house having a blood inlet, blood outlet, a dialysate inlet and a dialysate outlet, - a filter unit arranged within the house, said filter unit being formed by a plurality of hollow fibers, each hollow fiber having a predetermined length and a wall in the form of a semi-permeable membrane with a pore size of at most 1 micron; - wherein said plurality of hollow fibers provide a fluid communication between the blood inlet and the blood outlet, and the space outside the hollow fibers provides a separate fluid communication between the dialysate inlet and the dialysate outlet, - wherein the inner diameter of the hollow fibers is in the range of 100 to 400 microns, and - wherein at least a part of the inner surfaces of the fiber membrane adapted to contact the blood is coated with a binding agent for binding the CTCs flowing in the blood stream. - wherein said plurality of hollow fibers provide a fluid communication between the blood inlet and the blood outlet, and the space outside the hollow fibers provides a separate fluid communication between the dialysate inlet and the dialysate outlet, - wherein the inner diameter of the hollow fibers is in the range of 100 to 400 microns, and - wherein at least a part of the inner surfaces of the fiber membrane adapted to contact the blood is coated with a binding agent for binding the CTCs flowing in the blood stream.