Table 2 Masaoka-Koga staging system of thymoma

I

Macroscopically and microscopically encapsulated tumour

IIa

Microscopic invasion through the capsule

IIb

Macroscopic invasion into surrounding fatty tissue

III

Invasion into a neighbouring organ such as the pericardium, great vessels or lung

IVa

Pleural or pericardial dissemination

IVb

Lymphatic-haematogenous metastases