Introduction As a result of improving survival rates, the adverse consequences of rectal cancer surgery are becoming increasingly recognised. Low Anterior Resection Syndrome (LARS) is one such consequence and describes a constellation of bowel symptoms after rectal cancer surgery which includes urgency, faecal incontinence, stool clustering and incomplete evacuation. LARS has a significant adverse impact on Quality-of-Life (QoL) and symptoms are present in up to 75% of patients in the first year after surgery. Despite this, little is known about the natural history and there is poor evidence to support current treatment options. Methods and Analysis The objectives of POLARiS are to explore the natural history of LARS and to evaluate the clinical and cost-effectiveness of trans-anal irrigation (TAI) or sacral neural modulation (SNM) compared to optimised conservative management (OCM) for people with major LARS. All patients who have had an anterior resection in the last 10 years who meet the eligibility criteria will be invited to participate in the cohort. Patients identified as having a major LARS score (LARS score ≥30) and who meet the eligibility criteria will be invited to take part in the randomised controlled trial (RCT). Cohort and RCT participants will be followed up for a 24-month period, and will complete a series of questionnaires measuring LARS symptoms and quality-of-life, as well as clinical review for those in the RCT. A process evaluation, qualitative sub-study and economic evaluation will also be conducted. The primary outcome measure of the POLARiS cohort and RCT is the LARS score at 24 months. Analyses of the RCT will be conducted on an intention-to-treat basis. Comparative effectiveness analyses for each endpoint will consist of two pairwise treatment comparisons: TAI vs OCM and SNM vs OCM. Ethics and Dissemination Ethical approval has been granted by Wales REC 4 (reference: 23/WA/0171) in the UK and Sydney Local Health District HREC (reference: 2023/ETH00749) in Australia. The results of this trial will be disseminated to participants upon request and published on completion of the trial in a peer-reviewed journal and at international conferences Trial Registration Number ISRCTN12834598 Registered 04/08/2023 ACTRN12623001166662 Registered 10/11/2023

All authors were required to provide ICMJE COI disclosure form, the following disclosures were made: Professor Stocken is fully funded through the NIHR as a Research Professor and is an NIHR Efficacy and Mechanism Evaluation Funding Committee member. Professor Knowles has carried out speaker and consultancy work for Medtronic and consultancy work for Exero Medical. He is Chief Medical Officer and co-founder of Amber Therapeutics, and CMO and shareholder at Enterika LTD. Professor Quyn is a member of ACPGBI Advanced Cancer sub-committee and has received a ZR MedTech Honoraria for lecturing at the Chinese Colorectal Society. Mrs Cornish has received an educational grant from BD & Medtronic and is Clinical Director of Everywoman Festival. All other authors made no declarations.

ISRCTN12834598 Registered 04/08/2023 ACTRN12623001166662 Registered 10/11/2023

This work is supported by National Institute for Health Research (NIHR) Health Technology Assessment (HTA) Programme (Grant Ref: NIHR134937) in the UK and NHMRC-NIHR Collaborative Research Grant in Australia (Ref: 2015501).

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

This trial was reviewed and approved by Wales REC 4 (reference: 23/WA/0171) in the UK and Sydney Local Health District HREC (reference: 2023/ETH00749) in Australia. The appropriate local approval will be obtained by each participating site prior to entering participants into the trial. Further ethical approval of amendments to the protocol and/or related documents will be sought as necessary.

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I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

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I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

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All data produced in the present study are available upon reasonable request to the authors.