Chronic postsurgical pain (CPSP) has an incidence of 46% in patients after breast cancer surgery, which seriously affects patients' physiological and psychological function, as well as quality of life. Acute pain is an independent risk factor for persistent pain after surgery. Erector spinae plane block (ESPB) provided excellent perioperative analgesia in patients undergoing breast surgery. Dexmedetomidine as an adjuvant of local anesthetics prolongs the duration of peripheral nerve block and decreases the requirements of postoperative analgesia. The investigators hypothesize that, for breast cancer patients undergoing mastectomy, ESPB (with a combination of 0.5% ropivacaine 35 ml and dexmedetomidine 1 microgram/kg) can reduce the occurrence of CPSP. The purpose of this randomized controlled trial is to investigate the impact of ESPB with adjuvant dexmedetomidine on the incidence of CPSP in breast cancer patients after mastectomy. We will also observe the impact of ESPB on long-term survival in these patients.