Inclusion Criteria:  Signed informed consent form (ICF) indicating that the participant understands the purpose of, and procedures required for the study, and is willing to participate in the study Age ≥65 years Biopsy-confirmed squamous cell carcinoma (SCC) of the oral cavity, oropharynx, or hypopharynx (archived biopsies are allowed); if no biopsies are available, a new biopsy must be obtained to provide confirmation of SCC For participants with oropharyngeal cancer, human papilloma virus (HPV) p16 status must be known Tumor categories T3-T4 according to the 8th edition of the American Joint Committee on Cancer Staging Manual (AJCC v8) Has at least 1 tumor lesion that can be accurately measured according to RECIST 1.1 (per the Central Imaging vendor) and is amenable for intratumoral injection, as determined by the Investigator A single invaded, biopsy confirmed, accessible LN in the neck of ≥3 cm and <10 cm and with <180-degree encasement of the carotid artery on MRI or CT scan is eligible for intranodal injection If a LN is selected for injection, 1 of the 2 injected lesions must be the primary tumor itself  Ineligible to receive platinum-based chemotherapy for the treatment of LA HNSCC as defined by having at least one of the following:  Estimated creatinine clearance ≥30 and <50 mL/min (calculated by Cockcroft and Gault) Hearing loss or tinnitus Grade ≥2 Grade ≥2 peripheral neuropathy ECOG >2 Recent cardiac dysfunction (history of unstable angina pectoris, myocardial infarction, or New York Heart Association (NYHA) class III chronic heart failure <3 years prior to screening) Must be able to tolerate RT with curative intent as determined by the study Investigator. Not amenable to primary tumor surgery. Inability to undergo surgery must be determined by the treating physician in consultation with a head and neck surgeon, or if applicable, the site's multidisciplinary tumor board. ECOG performance status of 0 to 2 Life expectancy ≥6 months  Adequate organ and bone marrow function at screening as defined by:  Hemoglobin >9.0 g/dL Platelet count >100,000 cells/mm3 Leukocytes >3000 cells/mm3 Absolute neutrophil count >1500 cells/mm3 Alanine aminotransferase (ALT) ≤3×upper limit of normal (ULN) Aspartate aminotransferase (AST) ≤3×ULN Total bilirubin ≤1.5 mg/dL (in participants with Gilbert's syndrome, if total bilirubin is >1.5×ULN, measure direct and indirect bilirubin and if direct bilirubin is ≤1.5×ULN, the participant may be eligible) Total serum magnesium within normal ranges (1.7-2.2 mg/dL or 0.85 to 1.10 mmol/L)  Exclusion Criteria:  HNSCC category T1, T2 or M1 according to the 8th edition of the American Joint Committee on Cancer Staging Manual (AJCC v8) Has received prior antineoplastic systemic therapy or intervention (including pharmacological - both marketed and investigational, RT, or surgery) for the treatment of HNSCC Participants with known severe Grade 3 or 4 hypersensitivity reactions to cetuximab must be excluded from cetuximab treatment by the Investigator Known history of human immunodeficiency virus (HIV), active hepatitis B, or active hepatitis C infection Local regionally recurrent HNSCC Ulceration or other characteristics that may, in the opinion of the Investigator, increase the risk of severe tumor bleeding SCC originating in the nasopharynx or paranasal sinus, from the salivary gland, or thyroid gland, or non-squamous histology (e.g., melanoma or neuroendocrine carcinoma), or SCC of unknown primary origin Prior or concurrent malignancy whose natural history or treatment has the potential to interfere with the safety or efficacy assessment of the investigational regimen Clinically significant cardiac arrhythmias (e.g., ventricular tachycardia, ventricular fibrillation, torsades de pointes, second- or third-degree atrioventricular heart block without a permanent pacemaker in place) Class IV congestive heart failure as defined by the New York Heart Association functional classification system <6 months prior to screening A pregnant or nursing woman, or women of childbearing potential and men who are sexually active and not willing/able to use medically acceptable forms of contraception starting from signed ICF through 150 days after the last cetuximab dose/RT fraction. A woman who is 2 years postmenopausal or surgically sterile is not considered to be of childbearing potential. A known history of areca nut (betel nut) consumption Any condition for that, in the opinion of the Investigator, participation would not be in the best interest of the individual (e.g., compromises the participant's well-being) or that could prevent, limit, or confound the protocol/CIP specified assessments Subject participating in another clinical study at the time of signature of the informed consent form