This comparative study evaluates the short- and long-term outcomes of Quadrant-based Intersphincteric Resection (Q-ISR) versus traditional subtotal ISR (Sub-ISR) and conventional/total ISR (t-ISR) in patients with ultra-low rectal cancer undergoing sphincter-preserving surgery, with short-term endpoints focusing on perioperative safety (operative time, blood loss, length of stay, and postoperative complications such as Clavien-Dindo grade ≥II, anastomotic leakage/stricture, and stoma reversal) and long-term endpoints assessing anorectal function recovery (LARS and Wexner scores after stoma closure) and oncologic efficacy (R0 resection, recurrence patterns, and survival outcomes), aiming to determine whether a quadrant-tailored resection strategy can better balance tumor control with anal function preservation.

Inclusion Criteria:

• (1) Rectal cancer diagnosed by digital rectal examination, colonoscopy, and combined biopsy pathology. (2) The distance from the lower edge of the tumor to the dentate line is 1.5-2.0 cm. (3) Good anal function before surgery. (4) The tumor does not infiltrate the intersphincteric space. (5) The tumor length is less than 3 cm, and the proportion of the tumor occupying the intestinal lumen is less than 1/3 of the circumference. (6) American Society of Anesthesiologists (ASA) score is ≤3.

Exclusion Criteria:

• (1) Presence of distant metastasis. (2) Undifferentiated carcinoma or mucinous adenocarcinoma.