A 52-year-old man from Mumbai presents with a pigmented lesion on his back that has been growing over 6 months. Dermoscopy shows asymmetry, irregular borders, and mixed colors. Excision biopsy confirms cutaneous melanoma with a Breslow thickness of 2.8 mm, 3 mitotic figures per mm², and no ulceration. Which of the following is NOT a component of the AJCC TNM staging system for cutaneous melanoma?