Background: Lymphatic mapping identifies neck lymph nodes at risk for cancer spread in patients with lateralized oropharyngeal (OPC) squamous cell carcinoma. We hypothesize that a lymphatic mapping guided approach to radiotherapy (RT) treatment of the contralateral neck will enable safe de-escalation of therapy with acceptable disease control with potential benefits in RT related toxicity, QOL, swallowing function and economics. Methods: HN.11 is a Canadian Cancer Trials Group international multi-centre, non-inferiority randomized phase III trial comparing a lymphatic mapping-guided approach for management of the contralateral neck (experimental) vs. bilateral neck radiation therapy (RT) (control). The primary objective is to determine if a lymphatic mapping-guided approach for management of the contralateral neck has a non-inferior disease-free survival (DFS) compared to bilateral neck RT. Secondary objectives: To compare swallowing related QOL, xerostomia, isolated contralateral neck failure, overall survival, loco-regional failure, distant metastases, RT-related toxicities, patient reported adverse events (PRO CTCAE), gastrostomy tube usage, and economics indicators (resource utilization, lost productivity, financial toxicity, EQ5D). Exploratory objectives: Swallowing function using video fluoroscopic swallow studies, head and neck-cancer specific QOL, patterns of lymphatic drainage, radiomic prediction of contralateral lymphatic drainage, correlation of tumour somatic mutations and ctDNA with disease recurrence. Statistical Design: The target sample size is 510 patients. The experimental arm will be considered non-inferior if the upper limit of the one sided 95% confidence interval (CI) of the estimated hazard ratio (HR) does not exceed 1.46 (non-inferiority margin of 6.5% for 2-year DFS). In the primary intention-to-treat (ITT) analysis, the study has 81% power, with a one-sided type I error rate of 5%, assuming 5 years of accrual, 3-years of follow up to observe 178 DFS events, and a 5% loss to follow up. If non-inferiority is demonstrated in the ITT analysis, a secondary per-protocol analysis will be performed, analyzing those patients treated as per their allocation, with 80% power to detect non-inferiority of the experimental arm assuming a conservative 10% cross-over from the experimental to the control arm (i.e. failed lymphatic mapping). Conduct to Date: Study activation September 29, 2022. First enrollment was February 10, 2023. Supported by Canadian Cancer Society, Canadian Institutes of Health Research, National Cancer Trials Network (NCTN). Clinical trial information: NCT05451004.