Stage I includes: IA (T1a, N0, M0); IB (T1b, N0, M0). T1a: Solitary tumor equal to or less than 2 cm. T1b: Solitary tumor larger than 2 cm without vascular invasion. N0: No regional lymph node metastasis. M0: No distant metastasis. (AJCC 8th ed.) [from NCI]