Background: The optimal mode of resection for thymoma in nonmyasthenic patients remains unclear. The aim of this study was to explore whether or not thymomectomy alone is a relevant option for patients with stage I (T1N0M0) thymoma in the proposed TNM classification.

Methods: We investigated 2,835 patients with thymic epithelial tumors treated at 32 institutions participating in the Japanese Association for Research on the Thymus (JART). A total of 1286 patients with thymomectomy: resection of thymoma with partial thymectomy (n = 289) or thymothymomectomy: resection of thymoma with total thymectomy (n = 997) for stage I thymoma were included. Surgical and oncologic outcomes were compared between the 2 groups.

Results: Patients who underwent thymomectomy were older (61.1 versus 57.0 years; p = 0.000) and had smaller tumors (4.77 versus 5.99 cm; p = 0.000) than those who underwent thymothymomectomy. There was a significant difference in the distribution of histologic subtype (p = 0.007). After propensity-score matching, the matched cohort consisted of 276 patients in each group. Postoperative complications were seen more frequently in the thymothymomectomy group than in the thymomectomy group (8.3% versus 4.3%; p = 0.0397). The 5-year overall survival rate was 97.3% in the thymomectomy group and 96.9% in the thymothymomectomy group (p = 0.487). Patients who underwent thymomectomy tended to have local recurrence more frequently than did those who underwent thymothymomectomy (2.2% versus 0.4%; p = 0.0613).

Conclusions: Thymomectomy alone is acceptable for stage I thymoma in regard to postoperative complications and prognosis. Further studies are needed to evaluate long-term outcomes.