For many women with breast cancer, surgery is followed by chemotherapy administered into the bloodstream over several cycles. The need to receive chemotherapy may be confusing to the patient: Why should further treatment be given if the tumor has been removed? In general, surgery is very successful in getting rid of the initial cancerous growth. Tumor cells, however, can spread to other parts of the body such as the lymph nodes, internal organs, or bone—where they may not be detected until much later. Chemotherapy effectively reduces the risk that new tumors will arise from these hidden, potentially cancerous cells; studies have shown that it may diminish the likelihood of recurrent breast cancer in women by 20 to 37 percent. Unfortunately, there is a definite time frame in which the chemotherapy must be given in order to achieve optimal results, and this "window of opportunity" comes soon after surgery. Chemotherapy at this time has the advantage that tumor cells are still limited in number and have not developed into a full-blown tumor. Although chemotherapy is not very appealing to a patient so soon after the trauma of surgery, timely administration is crucial.