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The Osaka series that grouped thymomas by how far they had invaded, from fully encapsulated to spread inside the chest or beyond, and showed survival fell with each step. Its stages are still used forty years on.
Retrospective study of 96 patients with thymoma treated at Osaka University, proposing a four-stage system: stage I encapsulated, stage II invasion into the capsule or surrounding fat, stage III invasion of neighbouring organs, stage IVa pleural or pericardial dissemination and stage IVb lymphatic or haematogenous metastasis.
Five-year survival fell with stage, validating the system, which was later refined by Koga and remained the reference until the TNM classification of 2017 and is still reported alongside it.
Whether a thymoma is called stage I or stage III, and hence whether radiotherapy follows surgery, still traces back to this scheme.