Pathological snapshots of thymic epithelial tumors with invasion into neighboring structures: preparing for the forthcoming revision of the TNM classification.

Treatment decision-making of thymic epithelial tumors (TETs) after surgery is based on the pathological stage. Currently, most institutions use both the Masaoka-Koga system and the 8th edition of the tumor, node, metastasis (TNM) classification. Because these two systems separate each stage according to the same concept, namely, the "levels" of tumor extension, precise pathological evaluation of the presence or absence of tumor invasion into stage-defining structures is necessary. This review provides representative pathological snapshots of tumors invading neighboring structures to provide references that might be helpful to readers; the snapshots will cover features that correspond to those of "locally advanced TETs", the topic of this series. Tumor subtype, whether thymoma or thymic carcinoma, is another factor influencing treatment decisions. Accumulating evidence has indicated that most thymomas and thymic carcinomas have biologically distinct features. Representative results were achieved by a study conducted as part of The Cancer Genome Atlas (TCGA) project, and subsequent studies with the help of the TCGA data have further reported on these distinctive features. Here, we also introduce newly recognized features of TETs, mainly focusing on the difference between epithelial-rich thymomas and thymic squamous cell carcinoma. The new (9th) edition of the TNM classification will be launched in January 2024. Therefore, sharing current pathological features of TETs will help readers, not only in their daily practice but also in preparing for the upcoming classification system.