A 68-year-old man from rural Maharashtra presents with a non-healing ulcer on the dorsum of his left hand for 8 months. He is a farmer with lifelong sun exposure. On examination, the ulcer is 2 cm × 1.5 cm with raised, everted edges, a granulating base, and surrounding erythema. There is palpable regional lymphadenopathy. Dermoscopy shows a hyperkeratotic lesion with irregular vascular pattern. Biopsy confirms well-differentiated squamous cell carcinoma with invasion into the dermis. Which of the following is the most important prognostic factor in this patient?