Stage I includes: (T1, N0, M0); (T2, N0, M0). T1: Tumor invades the submucosa (through the muscularis mucosa but not into the muscularis propria). T2: Tumor invades the muscularis propria. N0: No regional lymph node metastasis. M0: No distant metastasis. (AJCC 8th ed.) [from NCI]