Unencapsulated thymoma: a case report.

Thymomas, the most common mediastinal tumors, form capsules. Only a few reports have presented small thymomas without capsule formation, so-called microthymomas. Here, we report a case of an unencapsulated thymoma measuring 18 mm. A 42-year-old female presented with an anterior mediastinal tumor. Computed tomography revealed an 18-mm nodule in the anterior mediastinum. Magnetic resonance imaging revealed a solid tumor that was iso-intense on T1-weighted images and hypo-intense on T2-weighted images. Thoracoscopic partial thymectomy was performed. The histopathological diagnosis was a type B1 thymoma. The tumor was localized within the thymic tissue lacked a fibrous capsule and partially invaded the surrounding fat tissue. To our knowledge, this is the first report of an unencapsulated thymoma, except for microthymomas.