A 58-year-old woman presents with a 6-week history of a rapidly enlarging, painful mass in the left submandibular gland region. On examination, the mass is 3.5 cm, firm, fixed to underlying tissues, and the overlying skin appears erythematous. There is mild ipsilateral cervical lymphadenopathy. FNAC shows malignant cells consistent with adenocarcinoma. What is the most appropriate next step in management?