This prospective randomized controlled double blinded non-inferiority study will be carried out in Tanta University Hospitals for duration of 6 months from June 2026 to December 2026 on 60 female patients scheduled for unilatral modified radical mastectomy under general anesthesia.

Breast carcinoma is a common malignancy in females, with modified radical mastectomy (MRM) being the most frequently performed surgery. Surgical intervention is accompanied by significant challenges such as postoperative pain and stress, potentially leading to prolonged hospitalizations and complications. Previous studies had demonstrated that thoracic paravertebral block (TPVB) was effective in diminishing opioid requirements for patients undergoing breast surgery. TPVB is technically challenging, requiring skilled healthcare professionals and the narrow paravertebral space, located between the superior costotransverse ligament (SCTL) and the parietal pleura, poses a risk of inadvertent pleural puncture and vascular damage, and increased the potential for pneumothorax and haematoma. The Subtransverse process Interligamentary (STIL) plane block is a recently introduced technique that holds promise as an alternative to TPVB. STIL plane block specifically targets the region adjacent to the paravertebral space, rather than directly penetrating it, which theoretically diminishes the potential risks of inadvertent pneumothorax and hematoma compared with TPVB. Additionally, due to its close anatomical proximity to the paravertebral space, the STIL plane block may facilitate a more straightforward dispersion of local anaesthetics into this area. Research has also confirmed that the block achieved with STIL plane block is effective in providing adequate pain relief for breast surgeries. Also it was associated with decreased postoperative opioid consumption compared to the ESP block in patients undergoing MRM. With the improvement of anaesthesia technology, the quality of patient recovery is no longer solely measured by the success of the surgery or the discharge rate, but is increasingly oriented towards a "patient-centered" approach that focuses on the safety, comfort, and overall quality of recovery throughout the perioperative period. In light of the growing emphasis on improving postoperative quality of recovery while ensuring patient safety, we hypothesize that the STIL plane block will demonstrate non-inferiority to the thoracic PVB in terms of postoperative quality of recovery and analgesia in patients undergoing MRM.

Inclusion Criteria:

• Adult female patients aged 18 to 65 years with American Society of Anesthesiologists (ASA) Physical Status I - III, who were scheduled for unilateral MRM surgery under general anesthesia and able to understand and complete the QoR-15 questionnaire

Exclusion Criteria:

• • Patients declined to participate in the trial.

  • History of allergy to the used local anesthetic (LA) drugs.
  • Patients with cutaneous lesions at the needle entry point.
  • Coagulopathy or current anticoagulant therapy contraindicating regional block.
  • Patients with severe hepatic, renal, cardiac, or respiratory disease.
  • Psychiatric or cognitive disorder interfering with QoR-15 assessment
  • Chronic opioid use or chronic pain syndrome.
  • Failed block.
  • Morbid obesity (body mass index >40 kg/m2)
  • Intraoperative major complications requiring postoperative ICU admission.
  • Pregnancy

Only participants who self identity as women are eligible to participate

The data will be available upon a reasonable request from the corresponding author

The data will be available upon a reasonable request from the corresponding author after the end of the study for 1 year