The present study is designed as a single-center, randomized controlled clinical study conducted at Keimyung University Dongsan Medical Center, a tertiary referral hospital in Daegu, Republic of Korea. The study investigates the efficacy and safety of bowel stimulation with probiotics before ileostomy closure in patients with rectal cancer who received preoperative chemoradiation and underwent low anterior resection with ileostomy creation, compared to patients who did not receive probiotics. The protocol has been developed in accordance with the Standard Protocol Items: Recommendations for Interventional Trials (SPIRIT) checklist [19].

Patients with rectal cancer who have undergone low anterior resection and a diverting ileostomy following chemoradiation and are scheduled for ileostomy closure will be enrolled. The investigator will provide detailed information about the study’s objectives, interventions, potential benefits, risks, and participant rights, allowing sufficient time for consideration. Patients will receive an informed consent form and may voluntarily sign it. Consent will also be obtained for reviewing medical records and collecting blood samples for recurrence assessment. Declining participation will not affect treatment, and patients may withdraw from the study at any time.

Inclusion criteria were as follows: (1) age 18‐80 years; (2) histologically confirmed rectal adenocarcinoma; (3) clinical stage II or III disease; (4) completion of neoadjuvant chemoradiotherapy; (5) prior laparoscopic or robotic low anterior resection with total mesorectal excision and creation of a diverting ileostomy; and (6) planned elective ileostomy closure after confirmation of anastomotic integrity.

Exclusion criteria were as follows: (1) suspected or confirmed inflammatory bowel disease; (2) evidence of anastomotic leakage, stricture, or distal obstruction precluding ileostomy closure; (3) medical contraindication to surgery; (4) severe immune-compromised status or uncontrolled systemic illness; and (5) inability or unwillingness to provide informed consent.

Participants will be randomly assigned to either study group in a 1:1 ratio using a computer-generated allocation sequence. To ensure allocation concealment, the sequence will be secured in sequentially numbered, opaque envelopes, open only after participant enrollment. These envelopes will be prepared by a study staff member independent of patient enrollment and data collection. To reduce bias, surgeons performing the operations will remain blinded to group assignment until after anesthesia is initiated.

Approximately one month prior to surgery, patients will undergo preoperative evaluations, including an abdominopelvic computed tomography (CT) scan, a loopogram with rectal iodinated contrast, a digital rectal examination, and rectoscopy. During rectoscopy, any changes in the mucosa or anastomosis site will be meticulously documented. If such changes are observed, biopsies will be taken as part of standard care for patients with rectal cancer during oncological follow-up. A comprehensive preoperative clinical and anesthetic evaluation will also be conducted.

The patients will receive a 14-French Foley catheter, normal saline, and probiotics (Lacidofil®) containing Lactobacillus rhamnosus R0011 and Lactobacillus acidophilus R0052 bacterial culture (2×109 colony-forming units) in an outpatient setting. This formulation was considered appropriate for the present trial because Lactobacillus-based probiotics have been investigated in colorectal and other gastrointestinal settings as microbiota-modulating interventions, although evidence remains inconclusive regarding their effect on postoperative bowel function and LARS [20]. The probiotic dose was selected to ensure a practical and standardized daily regimen for outpatient administration while remaining aligned with the commercially available formulation and with doses that have been evaluated in prior studies using the same product or closely related probiotic preparations [21]. The intervention group will receive 250 mL of normal saline with 4 g of Lacidofil® administered via the distal limb of the ileostomy once daily for 2 weeks prior to ileostomy closure. The control group will receive 250 mL of normal saline without probiotics using the same method. The 250 mL of normal saline will be infused over 10‐15 minutes via a 14-French Foley catheter with the patient in a supine position. Adherence will be monitored through daily patient diaries and weekly outpatient visits. Patients will record abdominal symptoms, including pain, nausea, vomiting, diarrhea, and gas and stool passage during the 2-week period before ileostomy closure.

Three experienced surgeons, each having performed over 50 ileostomy closure procedures, will conduct the surgeries. All patients will consume two 200 mL cans of a complex carbohydrate drink 2 hours before surgery and fast thereafter, receiving a second-generation cephalosporin as prophylactic antibiotic therapy. Ileostomy closure will be performed as follows:

Oral intake will begin the day after ileostomy closure unless obstructive symptoms are reported. Hospital discharge will be determined by the surgeon based on adequate pain control without the need for intravenous or intramuscular analgesics, tolerance of a soft diet, independent ambulation, and absence of significant complications or other medical concerns. Patients will be followed for at least 3 months post-surgery, with visits at 1 week and 1 month. At each visit, patients will be assessed and questioned regarding food acceptance via patient-reported dietary intake logs, stool habits using the Bristol Stool Chart and daily frequency records, and laboratory tests. Three months post-surgery, patients will complete the LARS questionnaire in an outpatient setting. The detailed schedule of assessments is presented in Figure 1.

The primary outcome is the LARS score at 3 months after ileostomy closure. The LARS score is a validated patient-reported outcome measure based on 5 items: incontinence for flatus, incontinence for liquid stools, bowel frequency, clustering of stools, and urgency. The total score ranges from 0 to 42, with higher scores indicating more severe bowel dysfunction. Scores are categorized as no LARS (0‐20), minor LARS (21-29), and major LARS (30-42). The questionnaire will be administered in the outpatient clinic at 3 months postoperatively.

The secondary outcomes include (1) postoperative complications within 30 days after ileostomy closure, including ileus, surgical site infection, anastomotic-related complications, readmission, and reoperation; (2) severity of postoperative complications, graded according to the Clavien-Dindo classification; (3) bowel recovery parameters, including time to first flatus, time to first stool passage, and tolerance of oral intake; (4) stool habits, including stool frequency and stool form assessed using patient records and the Bristol Stool Chart; (5) laboratory findings obtained during follow-up; and (6) length of postoperative hospital stay.

Patients will enter an open follow-up phase for up to 5 years for routine postoperative surveillance, monitoring oncologic outcomes and late complications through periodic clinical visits and imaging studies per institutional guidelines.

The sample size calculation was based on the primary outcome, the LARS score at 3 months after ileostomy closure. Based on a previous study [21], we considered a 3-point between-group difference in LARS score to be clinically relevant. For the sample size estimation, the standard deviation was conservatively assumed to be 5 points. Using a 2-sided significance level of .05 and 80% power, 45 participants were required per group. Allowing a 10% dropout rate, the final target sample size was set at 100 participants (50 per group).

Results will be presented as means with standard deviations for continuous variables and as frequencies and percentages for categorical variables. The χ² test or Fisher exact test will be used for categorical data, while the Student’s t-test or Mann-Whitney U test will be applied to continuous data. Logistic regression will be conducted for univariate and multivariate analyses. For adjusted analyses, prespecified covariates will include age, sex, body mass index, tumor location, clinical stage, neoadjuvant treatment history, operative approach, anastomotic level, and the interval between index rectal surgery and ileostomy closure. Exploratory subgroup analyses may be performed according to sex, age group, operative approach, and anastomotic level. Because the study is not powered for subgroup comparisons, these analyses will be interpreted cautiously. A P value <.05 will be considered statistically significant.

Analyses will be conducted on an intention-to-treat basis, including all randomized participants in their assigned groups, regardless of protocol adherence. A per-protocol analysis will be performed as a sensitivity analysis to assess result robustness. For missing data, multiple imputation methods will be employed, assuming data are missing at random. Sensitivity analyses will evaluate the impact of missing data on primary and secondary outcomes. Statistical analyses will be conducted using SPSS software (version 25; IBM Corp., Armonk, NY, USA).

Data will be collected using a standardized case report form and entered into a Microsoft Access database. Collected data will include demographic characteristics, baseline clinical variables, perioperative findings, intervention adherence, patient-reported bowel symptoms, dietary intake logs, Bristol Stool Chart records, laboratory results, postoperative complications, and follow-up outcomes including the LARS questionnaire. Data accuracy will be monitored through double data entry and weekly audits.

Participant retention will be promoted through reminder calls and flexible outpatient visit scheduling. Regardless of findings, results will be submitted for publication in a peer-reviewed journal. A data monitoring committee is not required, as probiotics are classified as low-risk interventions per International Council for Harmonisation (ICH) Good Clinical Practice (GCP) guidelines and prior studies [12,15]. No interim analyses are planned, and the study will conclude upon data collection completion unless unexpected adverse events prompt a halt and review.

This study was approved by the Institutional Review Board of Keimyung University Dongsan Medical Center (IRB No. DSMC-2024-03-016). The study will be conducted in accordance with the ethical principles of the Declaration of Helsinki and applicable institutional requirements. Written informed consent will be obtained from all participants before enrollment and before any study-related procedures are performed. The investigator or designated research staff will explain the study objectives, procedures, potential risks and benefits, and participant rights, and sufficient time will be provided for questions and consideration.

Participant data will be d-identified before analysis and stored securely in a restricted-access setting. Access to identifiable information will be limited to the principal investigator and authorized research staff. No personally identifiable information will be disclosed in any publication or report arising from this study. Participants will not receive financial compensation for participation. However, participants who experience study-related harm or adverse effects will be managed and compensated in accordance with applicable institutional policies and regulations.

The protocol may be modified through agreement between the principal investigator and trial participants, with any changes requiring Institutional Review Board approval. Participants will be informed of amendments via written notification, and re-consent will be obtained if changes affect their participation.

Study results will be shared with investigators and patients, published in a peer-reviewed journal regardless of findings, with authorship following International Committee of Medical Journal Editors (ICMJE) guidelines. The full protocol will be publicly available via the Clinical Research Information Service (CRIS) registry, with deidentified data available upon reasonable request.

The study protocol received approval from the Institutional Review Board of Keimyung University Dongsan Medical Center (IRB No. DSMC-2024-03-016) and was registered with the Clinical Research Information Service, Republic of Korea (KCT0011052). Participant recruitment is scheduled to begin in March 2026 and is expected to continue through March 2029. The target sample size is 100 participants, with 50 participants allocated to each study group. Final data collection for the primary endpoint will be completed after the 3-month follow-up of the last enrolled participant, and the main study findings are expected to be published in 2029.

As for manuscript submission, participant enrollment had not yet started because the study was in the pre-enrollment phase following ethics approval and trial registration. Therefore, no participants had been randomized, no baseline data had been collected, and no outcome analyses had been performed. No protocol amendments, deviations, or interim analyses had occurred before manuscript submission.