Abstract
BACKGROUND AND OBJECTIVE: Thymic epithelial tumors (TETs) are the most common mediastinal neoplasms and include thymomas, thymic carcinomas, and thymic neuroendocrine neoplasms (TNENs). The staging system of TETs has been based on the Masaoka-Koga system or the 8th edition of the TNM classification, which does not consider tumor size as a T descriptor. The 9th edition of the TNM classification was released in January 2025, and tumor size was incorporated, with the T1 category subdivided into T1a (≤5 cm) and T1b (>5 cm). Thus, the clinical importance of tumor size in TETs has attracted increasing attention. This review summarizes previous reports focusing on tumor size as a prognostic factor for TETs and highlights the association between tumor size, prognosis, and clinicopathological features of TETs. METHODS: The literature search was performed using PubMed for the narrative review. Eligible articles were published in English between January 1, 2004 and December 1, 2024. KEY CONTENT AND FINDINGS: We identified 35 articles investigating the effect of TET tumor size. Tumor size assessed using surgical specimens was a useful predictor for prognosis in all stages of thymomas. A large tumor size was associated with tumor invasion into adjacent tissues, which contributes to advanced-stage disease and incomplete resection. Thus, large tumor size was shown to be related to a high recurrence rate and poor prognosis. In addition, tumor size had a strong prognostic impact in patients with early-stage thymoma. Consistent with the evaluation of surgical specimens, preoperative assessment of tumor size using computed tomography also contributed to the postoperative prognosis. Furthermore, evaluation of tumor size may help determine treatment strategies, such as surgical approaches and adjuvant radiotherapy. However, the prognostic role of tumor size in thymic carcinoma and TNENs was unclear because of their rarity. CONCLUSIONS: Primary tumor size was identified as an important prognostic factor in patients with thymoma. However, the significance of thymic carcinoma or TNEN remains unclear. Further prospective large-scale studies are warranted to investigate the clinical significance of tumor size in TETs.