A 58-year-old male tobacco chewer from rural Maharashtra presents with a 3-month history of a non-healing ulcer on the left buccal mucosa. On examination, the ulcer is 2 cm × 1.5 cm with indurated, everted margins and a granular base. The floor of the mouth appears elevated. Palpation reveals a firm, fixed mass. There is ipsilateral submandibular lymphadenopathy (2 cm, mobile). Biopsy confirms squamous cell carcinoma. Which of the following is the most important prognostic factor in this case?