A 58-year-old woman with a 10-year history of vitiligo presents with a solitary, rapidly enlarging, darkly pigmented nodule on the dorsal surface of her right foot. Dermoscopy reveals a polymorphous vascular pattern with irregular dots and globules. Histopathology shows nests of epithelioid melanocytes with moderate nuclear pleomorphism, scattered mitotic figures (8 per 10 high-power fields), and a brisk lymphocytic infiltrate at the base. The lesion measures 1.2 cm in maximum dimension. Which of the following is the single most important adverse prognostic factor in this case?