Background: Treatment for HPV positive(+) oropharyngeal squamous cell carcinoma (OSCC) is highly effective but associated with significant short and long term treatment related morbidity. We hypothesize that decreasing the regions of elective nodal irradiation (ENI) in the neck will lead to less toxicity and better quality of life/functional outcomes while maintaining high disease control rates in patients with favourable prognosis HPV+ OSCC. Methods: HN.10 is a Canadian Cancer Trials Group phase II trial with a primary objective to evaluate the efficacy of primary definitive radiotherapy (RT) or chemoradiotherapy (CRT) utilizing volume reduced ENI as measured by 2-year event-free survival (EFS) in patients with low-risk HPV+ OPSCC. Secondary objectives include to evaluate overall survival, local control, regional control, locoregional control, out-of-field regional control, distant metastasis free survival, early and late toxicities of treatment, subjective swallowing functions, quality of life, utilization of healthcare resources, work productivity, and prognostic biomarkers. An imaging and biospecimen bank will be compiled as part of trial conduct. Key eligibility criteria include: pathologically proven diagnosis of HPV+ OPSCC; HPV association determined locally by either p16 immunohistochemistry or direct detection of HPV DNA sequences (e.g. by PCR or in situ hybridization) performed on a core needle or surgical biopsy specimen of the primary tumour or involved cervical lymph node; clinical stage T1-3 N0-1 M0 (UICC/AJCC 8th Ed.); fit for radiotherapy +/-chemoradiotherapy. Statistical Design: The primary endpoint is 2-year EFS. Assuming 2-year EFS to be 91% (Ha) for low-risk HPV-related OPSCC with standard treatment, and that the experimental treatment will be considered as ineffective if the 2-year EFS is ≤ 85% (H0), with one-sided alpha of 0.1, a sample size of 100 patients will have 80% power to detect a 6% difference of 2-year EFS. With 3 years of accrual and 2 years of follow-up, the total duration of this study will be 5 years. A total of 304.7 person-years of follow-up is needed for the final analysis. The null hypothesis (H0) will be rejected when the observed survival rate is 88.85% or higher (i.e. if there are 18 or fewer EFS events observed). Conduct to Date: Study activation February 20, 2019. Enrollment as of January 29 2020: 23. Clinical trial information: NCT03822897.