Stage I Thymoma AJCC v9

**Semantic type:** Neoplastic Process

**Definition:** Stage I includes: (T1a, N0, M0); (T1b, N0, M0). T1a: Tumor measuring 5 cm or less in greatest dimension with either: limited to the thymus with or without encapsulation; directly invades into the mediastinal fat only; directly invades the mediastinal pleura but does not involve any other mediastinal structure. T1b: Tumor measuring more than 5 cm in greatest dimension with either: limited to the thymus with or without encapsulation; directly invades into the mediastinal fat only; directly invades the mediastinal pleura but does not involve any other mediastinal structure. N0: No involvement of regional lymph node(s). M0: No distant metastasis. (AJCC 9th ed.)