The development of personalized cancer therapy depends on a robust system to monitor the patient's individual response to anticancer treatment. Anticancer drug efficacy has been tested on circulating tumor cells (CTCs) derived from patient blood samples after ex vivo expansion into CTC clusters. Current attempts to culture these primary cancer cells focus on long-term maintenance under growth factor supplements into cell lines, which usually takes >6 months and results in a CTC expansion efficiency of <20%. We recently developed a simple but unique microfluidics-based culture approach that requires minimal preprocessing (∼30 min) and does not require prior enrichment of CTCs or depend on the use of growth factor supplements. The approach capitalizes on co-culture of immune cells from the same patient blood sample within specially designed microwells that promote CTC cluster formation within 2 weeks, with an overall cluster formation success rate of ∼50%. Drug screening is facilitated by the incorporation of a gradient generator for parallel exposure to two or more drugs at various concentrations. Owing to the cost-effectiveness and less-invasive nature of this procedure, routine monitoring of disease progression can be achieved. The described microfluidics system can be operated with a single syringe pump to introduce drug compounds (which takes ∼6 min), followed by incubation of the CTC clusters for 48 h before analysis. In addition to its applications in biomedical research, the rapid readout of our platform will enable clinicians to assess or predict a patient's response to various therapeutic strategies, so as to enable personalized or precision therapy.