A device can be used to retain circulating tumor cells (CTCs). The device can include a cross-flow module with a retentate channel and a permeate channel. A filter in the cross-flow module can separate the retentate channel from the permeate channel. The filter can be constructed such that CTCs are retained in the retentate channel while other cells can pass through the filter into the permeate channel. A recirculation channel can direct a flow from an outlet of the retentate channel back to an inlet of the retentate channel to thereby concentrate CTCs in the retentate flow.
