Developed in 1981 by Akira Masaoka and updated by Kenji Koga in 1994, the Masaoka-Koga staging system is regarded as the best indicator of thymoma malignancy. Stage 1 indicates total encapsulation. Stages IIA and IIB indicate the thymoma has spread microscopically or macroscopically to the thoracic fatty tissue. Stage III indicates spread to the lungs, pericardium, and neighboring organs. Stages IVA and IVB indicate metastasis to the membranes of the lungs, pleura, and heart, or to the blood and lymph nodes.

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