- Tumor histology and immunohistochemical studies:

- Tumor extension: Encapsulated tumor or capsule missing but without invasion into surrounding tissue Microscopic transcapsular invasion into thymic or perithymic fat Macroscopic invasion into thymic or perithymic fat Confined to thymus Invades mediastinal pleura Invades pericardium Invades diaphragm Lung, specify lobe: Brachiocephalic vein Superior vena cava Phrenic nerve Chest wall Extrapericardial pulmonary artery Extrapericardial pulmonary veins Ascending aorta Aortic arch Descending aorta Arch vessels Intrapericardial pulmonary artery Myocardium Trachea Esophagus Other (specify): Cannot be assessed No applicable No evidence of primary tumor

- Surgical margins: Cannot be assessed Uninvolved by tumor, specify: , closest distance: ,and those margins <2 cm: Involved by tumor, specify:

Stage I: Grossly and microscopically encapsulated (includes microscopic invasion into, but not through, the capsule) Stage IIa: Microscopic transcapsular invasion Stage IIb: Macroscopic capsular invasion Stage III: Macroscopic invasion of neighboring organs Stage IVa: Pleural or pericardial dissemination Stage IVb: Hematogenous or lymphatic dissemination Cannot be determined

+ Great vessel involvement (for Stage III): Not identified Present Cannot be determined

Age-appropriate involution changes Fibrosis Follicular thymic hyperplasia Epithelial thymic hyperplasia True thymic hyperplasia Cystic changes in tumor Cystic changes in adjacent thymus

- Other (specify):