Masaoka-Koga Staging for Thymoma with Macroscopic Capsular Invasion
Macroscopic invasion into the capsule of a thymoma corresponds to Stage IIB in the Masaoka-Koga staging system. 1
Staging Breakdown
The Masaoka-Koga staging system distinguishes between microscopic and macroscopic capsular invasion at the Stage II level:
Stage I
- Grossly and microscopically completely encapsulated tumor
- Invasion into but not through the capsule
- 10-year overall survival: 84% 1
Stage IIA
- Microscopic transcapsular invasion (defined as <3 mm by ITMIG refinements)
- 10-year overall survival: 83%
- 10-year recurrence rate: 8% 1
Stage IIB
- Macroscopic invasion into thymic or surrounding fatty tissue
- Grossly adherent to (but not breaking through) mediastinal pleura or pericardium
- Gross extension into normal thymus or perithymic fat
- Adherence to pleura or pericardium with microscopic confirmation of perithymic invasion 1
Stage III
- Macroscopic invasion into neighboring organs (pericardium, great vessels, lung)
- Microscopic invasion of mediastinal pleura or pericardium
- Invasion into phrenic or vagus nerves
- 10-year overall survival: 70%
- 10-year recurrence rate: 29% 1
Key Clinical Distinction
The critical distinction is that Stage IIB involves macroscopic invasion beyond the capsule into perithymic structures, while Stage III involves invasion into actual neighboring organs (pericardium, great vessels, lung parenchyma). 1
Prognostic Implications
Complete surgical resection remains the most significant prognostic factor for disease-free survival and overall survival, superseding the impact of tumor stage alone. 1 Stage I/II thymomas are generally considered resectable upfront and amenable to complete surgical resection. 1
Common Pitfall
Do not confuse macroscopic capsular invasion (Stage IIB) with macroscopic organ invasion (Stage III). Stage IIB involves extension into fat and adherence to pleura/pericardium without penetration, while Stage III involves actual penetration into organs or their serosal layers. 1