Abstract
Ectopic extrathoracic thymoma is an uncommon clinical entity that has only been described sporadically, and most of them are ectopic hamartomatous thymomas or ectopic cervical thymomas. Here a case of 32-year-old woman with an ectopic thymoma was presented, which was found in right armpit and a postoperative diagnosis of 2018 World Health Organization (WHO) confirmed type B1, modified Masaoka stage I tumor was obtained. The patient had no symptoms of myasthenia or other immune disorders but once misdiagnosed as supernumerary breast. Tumor resection was performed and the tumor was completely removed. To our knowledge, this is a novel report that ectopic thymoma exists on chest wall and type B1 was confirmed according to 2018 WHO histological classification.