Type B2 thymomas are more likely to invade beyond capsule into the surrounding fat (as shown here) or mediastinal structures as compared to thymomas type A, type AB, or type B1. Only about 32% of type B2 thymomas present in Stage I as compared to 50% of cases for type B1 thymoma. Staging: Thymomas are staged according to the Masaoka-Koga Classification System which combines histologic criteria with elements of the TNM classification. Stage I: Completely encapsulated grossly and microscopically. Stage II: Microscopic capsular invasion (Stage IIa) OR macroscopic invasion into thymic or surrounding adipose tissue, or gross adhesions to but no invasion of pleura or pericardium (Stage IIb). Stage III: Macroscopic invasion into surrounding structures (lung, pericardium, great vessels). Stage IV: Pleural or pericardial metastases (Stage IVa) OR distant metastases via lymphoid or hematogenous route (Stage IVb).