To study the clinicopathologic features, immunophenotype and differential diagnosis of atypical type A thymoma.

Clinicopathologic and follow-up data of three cases of atypical type A thymoma from 2004 to 2016 were reviewed. Immunohistochemical staining was performed.

All three patients were male with average age of 59 years. Clinically, the lesions presented as anterior mediastinal masses. Grossly, the tumor ranged in size from 4 to 6 cm in greatest dimension and partially enclosed within fibrous capsule. The cut surface was homogenously fleshy, tan to brown in color, with multinodular and lobulated configurations. Cystic changes and necrosis were seen. Microscopically, the tumor was composed of plump spindle or oval-shaped cells arranged in storiform, microcystic, glandular, rosettes-like and hemangiopericytoma-like histological patterns. The tumor cells showed mild cytological atypia and mitotic activity ranged from 4 to 5/10HPF. Necrosis was present in all tumors. All tumors showed diffuse CK(AE1/AE3), CK19, p63 and vimentin expression. TdT, CK20, CD20, CD5 and CD117 were negative. The proliferative index, as measured with Ki-67, was 15.2% to 26.4%. None of the cases had recurrence or metastases during the follow-up period (9 to 27 months).

Atypical type A thymoma presents atypical features including tumor necrosis and increased mitotic activity. However, the implications of such atypical features to the biological behavior of the tumor remain to be determined.

Thymoma; Diagnosis, differential; Atypical