Postoperative pain following modified radical mastectomy remains a significant clinical challenge and may delay recovery, impair patient comfort, and increase opioid consumption. Ultrasound-guided fascial plane blocks have become an important component of multimodal analgesia for breast surgery. The serratus anterior plane block (SAPB) is an established regional anesthesia technique that provides effective postoperative analgesia. The serratus posterior superior intercostal plane block (SPSIPB) is a recently described interfascial block with promising analgesic effects, but evidence comparing its efficacy with SAPB in breast surgery remains limited.

This prospective, randomized, double-blind, single-center clinical trial aims to compare the postoperative analgesic efficacy of ultrasound-guided SPSIPB and SAPB in patients undergoing modified radical mastectomy under general anesthesia. Seventy female patients aged 18-65 years with American Society of Anesthesiologists (ASA) physical status I-III will be randomly assigned to receive either SPSIPB or SAPB before surgery. The primary outcome is postoperative pain intensity assessed using the Visual Analog Scale (VAS). Secondary outcomes include intraoperative remifentanil consumption, cumulative tramadol consumption during the first 24 postoperative hours, postoperative nausea and vomiting (PONV), quality of recovery assessed with the QoR-40 questionnaire, rescue analgesic requirements, and block-related complications. The findings of this study are expected to provide evidence regarding the comparative effectiveness and safety of these two interfascial plane blocks for postoperative analgesia in breast cancer surgery.