A 72-year-old man with a history of chronic sun exposure presents with a 1.8 cm firm, flesh-colored nodule with central ulceration on his lower lip. He reports the lesion has been present for 6 months and has not responded to topical antibiotics. On examination, the lesion is indurated with rolled edges and a granulating base. There is mild erythema and scaling at the margins. Histopathology confirms moderately differentiated squamous cell carcinoma with invasion into the submucosa. Immunohistochemistry shows high Ki-67 expression. What is the most important prognostic factor that will influence his treatment and follow-up strategy?