A 52-year-old woman with a history of tobacco and betel nut chewing presents with a 4-month history of a painless swelling on the floor of mouth. On examination, there is a 2.5 cm × 2 cm firm, non-mobile mass with overlying erythema. Biopsy confirms moderately differentiated squamous cell carcinoma. Clinical examination shows no cervical lymphadenopathy. Contrast-enhanced CT reveals a T2N0M0 tumor. What is the most appropriate next step in management?