This prospective randomized double-blind controlled trial aims to compare the postoperative analgesic efficacy and dermatomal spread of rhomboid intercostal-subserratus plane (RISS) block and serratus posterior superior intercostal plane (SPSIP) block in patients undergoing modified radical mastectomy. Postoperative pain scores, opioid consumption, rescue analgesic requirements, block-related complications, and quality of recovery outcomes will be evaluated.

Breast cancer surgery is frequently associated with moderate to severe postoperative pain, which may negatively affect recovery and increase opioid consumption. Ultrasound-guided fascial plane blocks have become an important component of multimodal analgesia strategies for breast surgery.

The rhomboid intercostal-subserratus plane (RISS) block and serratus posterior superior intercostal plane (SPSIP) block are recently described regional anesthesia techniques that may provide effective postoperative analgesia by blocking the lateral and posterior branches of the thoracic spinal nerves. However, comparative clinical evidence regarding their analgesic efficacy and dermatomal coverage in modified radical mastectomy remains limited.

This prospective randomized double-blind controlled trial will compare RISS block and SPSIP block in adult patients undergoing modified radical mastectomy under general anesthesia. Participants will be randomly allocated to receive either RISS block or SPSIP block before surgery. The primary objective is to compare postoperative opioid consumption. Secondary outcomes include postoperative pain scores, dermatomal sensory spread, time to first rescue analgesia, quality of recovery, opioid-related adverse events, and block-related complications.

Inclusion Criteria:

• Female patients aged 18-75 years
• ASA physical status I-III
• Scheduled for elective unilateral modified radical mastectomy with axillary dissection under general anesthesia
• Provision of written informed consent

Exclusion Criteria:

• Anticoagulant therapy
• Bleeding diathesis
• Known allergy to local anesthetics or opioids
• Infection at the block application site or previous surgery in the block area
• Cognitive impairment preventing NRS assessment
• Pregnancy or lactation
• Chronic opioid use
• Severe hepatic or renal failure
• Refusal to participate in the study
• Technical inability to perform the block or failed block

Participants and outcome assessors will be blinded to group allocation. Regional blocks will be performed by an anesthesiologist not involved in postoperative assessments.