The Phase III study will include 174 patients with locally advanced oropharynx carcinoma, receiving all arc-IMRT (Intensity-Modulated Radiation Therapy) (70 Gy) with concomitant systemic therapy. Two arc-IMRT treatment arms will be compared: one "standard" arm based on the use of a single pre-treatment planning and one "experimental" arm (adaptive RT) based on a weekly replanning to spare the salivary glands. The main objective is to increase by 25% the salivary flow (Parafilm) 12 months after RT thanks to adaptive RT, while not decreasing local control. The secondary objectives are to increase the salivary flow (scintigraphy), reduce xerostomia, acute and late toxicities (Eisbruch questionnaire, MDAS-HN, v.4 CTCAE), while maintaining local control (stopping rule of the trial if difference>15%). 174 patients will be included in 6 French centers for 2 years and followed for 2 years. The HPV (Human Papillomavirus) status will be identified and the tumors frozen. A central IMRT QA (Quality Assurance) will be performed.

Inclusion Criteria:

1. Locally advanced non-metastatic carcinoma of the oropharynx limited to T3 and T4 (whatever the N) and N2-N3 (whatever the T)(AJCC stage III-IV)
2. Age ≥ 18 years and ≤ 75 years
3. Performance status (WHO ≤ 2)
4. Renal, hepatic and cardiovascular functions allowing systemic treatment administration
5. Adapted stomatologic care
6. Signed informed consent form
7. Membership or beneficiary of a national insurance scheme

Exclusion Criteria:

1. Both parotids totally included in the target volume
2. Stages T1 or T2 with positive node disease N1
3. Neoadjuvant chemotherapy
4. Exereses of primitive tumor and/or nodes
5. History of other cancer within 5 years (except for basocellular epithelioma and cervical)
6. Previous neck radiotherapy
7. Platinum salts, 5FluoroUracile (5FU) agents or cetuximab chemotherapy considerations
8. Unstable diseases (cardiovascular, renal, pulmonary, systemic lupus or sclerodermia)incompatible with study participation
9. Patient participating in other therapeutic trial with experimental drug, 30 days before the inclusion.
10. Patient already recruited in another biomedical research ( non interventional study is authorized)
11. Pregnant or breast feeding patients
12. Patient deprived of liberty, under tutorship, guardianship or placed under judicial protection
13. Patient is deemed incapable of giving informed consent
14. Patients who, for family, social, geographic or psychological reasons, cannot be adequately followed up and/or are incapable of undergoing regular controls.