Type B3 thymomas are more likely to present at advanced stages than other types of thymomas and may be unresectable. They frequently invade beyond capsule into the surrounding fat (Stage IIa; as shown here) or adjacent mediastinal structures. 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).