SPECT-CT Guided ELEctive Contralateral Neck Treatment in Lateralized Oropharyngeal Cancer

Inclusion Criteria: 1. Patients must have signed a written informed consent form prior to any trial specific procedures. 2. Patients with histologically confirmed T1-T3 M0 lateralized OPC (tonsil, soft palate, pharyngeal wall or base of tongue) not involving or crossing midline. Nodal disease may include no node or single or multiple ipsilateral lymph nodes (largest should be equal or less than 6 cm in maximum diameter) without contralateral nodes involved. For HPV-positive patients, this includes N0-N1. For HPV-negative patients, this includes N0-N2b. Patients with radiologic extranodal extension without clinical signs of extranodal extension (skin invasion, deep nodal fixation, and/or clinical signs of cranial nerve or brachial plexus invasion) will be eligible for participation. 3. HPV-positive or -negative (by p16 immunohistochemistry). Tumours will be classified as p16 at local sites based on greater than 70% strong diffuse nuclear or nuclear and cytoplasmic staining. 4. Planned definitive bilateral neck radiotherapy with or without concurrent chemotherapy. 5. Patients ≥ 18 years old. 6. ECOG Performance Status 0-1. 7. The following radiological investigations must have been done within 8 weeks before randomization: * CT or MRI of the neck (with head imaging as indicated); * PET-CT scan; * Chest CT scan. 8. Patients who receive a concomitant chemoradiotherapy (cCRT) should have adequate organ and bone marrow function including the following: * Hematological function (absolute neutrophil count ≥ 1.5 x10⁹/L, platelets ≥ 100 x10⁹/L, hemoglobin ≥ 9 g/dL) measured before cCRT. * Renal function (creatinine clearance ≥ 50 mL/min per Cockcroft and Gault formula) measured before cCRT. * Hepatic function (total bilirubin \< 1.5 ULN, Aspartate aminotransferase (AST) and Alanine aminotransferase (ALT) \< 2.5 ULN, Alkaline phosphatase \< 2.5 ULN) measured before cCRT. 9. Women/men of childbearing potential must have agreed to use a highly effective contraceptive method up to 90 days after completing radiotherapy. Women of childbearing potential must have a negative pregnancy test before the beginning of the trial. 10. Treating surgeon must confirm that the patient is a candidate to undergo injection procedure for lymphatic mapping in either the nuclear medicine, ambulatory clinic, or operating room setting. 11. Patient is willing and able to comply with the protocol for the duration of the trial including undergoing treatment and scheduled visits, and examinations including follow-up. 12. Patients affiliated to (or beneficiary from) the French social security system. 13. Patients with a prior or concurrent malignancy whose natural history or treatment does not have the potential to interfere with the safety or efficacy assessment of the investigational regimen are eligible for this trial. Exclusion Criteria: 1. Patients with T1-T2 cancers isolated to the tonsil fossa (i.e., without any soft palate, tongue base, posterior pharyngeal wall or posterior tonsil pillar involvement) with no involved lymph nodes or with a single ipsilateral node \< 3 cm without extranodal extension. 2. Patients with tonsil or tongue base primary squamous cell carcinoma who have previously undergone diagnostic palatine or lingual tonsillectomy with either complete excision or with no clinically apparent residual disease are excluded. However, patients who have had previous deep biopsies or partial excisions with clinically evaluable disease are still eligible. 3. Previous head and neck cancer or multiple synchronous primary head and neck cancers. 4. Previous induction or neo-adjuvant chemotherapy. 5. Previous radiation therapy to the head and neck or comprehensive neck dissection of at least 3 levels on either side (due to potential for disrupted lymphatic channels and drainage pathways). Patients who have had excisional biopsies of involved lymph nodes are, however, still eligible. 6. Previous radiotracer allergy. Contraindication in patients with history of hypersensitivity to human albumin-containing products. 7. Patients with severe, active co-morbidity including any of the following: * Chronic Obstructive Pulmonary Disease or other pulmonary illness requiring hospitalization within 30 days of registration. * Unstable angina and/or congestive heart failure requiring hospitalization within the 30 days of registration. * Acute myocardial infarction within 30 days of study registration. * Diseases precluding RT (e.g., scleroderma). 8. Presence of any psychological, familial, sociological or geographical condition potentially hampering compliance with the study protocol and follow-up schedule, as assessed by the investigator. 9. Pregnant or breastfeeding women. 10. Patient enrolled in another therapeutic trial within 30 days of registration. 11. Persons deprived of their liberty or under protective custody or guardianship.