A 68-year-old farmer from rural Maharashtra presents with a non-healing ulcer on the dorsum of his left hand for 8 months. He reports the lesion started as a small erythematous nodule that gradually enlarged and ulcerated. On examination, there is a 2 cm × 1.5 cm indurated ulcer with everted edges, surrounding erythema, and regional lymphadenopathy. Dermoscopy shows hyperkeratosis and abnormal vascular patterns. A biopsy confirms well-differentiated squamous cell carcinoma with invasion into the dermis. Which of the following is the most important prognostic factor for this patient?