Clinical history: 54 year-old male with atrial fibrillation. Incidentally found to have a partially cystic anterior mediastinal mass.
What is the diagnosis?
Type A thymoma
Thymomas are neoplasms arising from or exhibiting differentiation towards thymic epithelial cells, regardless of the presence and relative numbers of non-neoplastic lymphocytes. Thymomas are classified based on whether the neoplastic epithelial cells and their nuclei have a spindle or oval shape, and are uniformly bland (type A) or whether the cells have a predominantly round or polygonal appearance (type B). Type B thymomas are further divided on the basis of the extent of the lymphocytic infiltrate and the degree of atypical of the neoplastic epithelial cells into three subtypes: B1- richest in lymphocytes, B2, and B3- richest in epithelial cells. Thymomas combining type A with B1 or B2-like features are designated type AB.
Type A thymoma is relatively uncommon and accounts for 4-19% of all thymomas. The age at manifestation ranges from 32-83 years with a mean age of 61 years which is higher than the mean age of 50 years of all thymoma patients. There is no consistent gender predilection. Approximately 24% of type A thymomas are found in patients with myasthenia gravis.
Grossly, type A thymomas is usually well circumscribed and encapsulated. The cut surface is white and cystic change or calcification of the capsule may be seen. Average tumour size is 10.5 cm.
Histologicaly, the tumour has few or no lymphocytes and shows neither distinct lobules nor dissecting fibrous bands as seen in other types of thymoma. The tumour cells are spindle with bland nuclei, dispersed chromatin and inconspicuous nucleoli. The cells are arranged in solid sheets without any particular pattern or in a storiform pattern. Vessels in the background may impart a hemangiopericytoma-like appearance. Cells in mitosis are seldom found. Thymic carcinoma may rarely arise in Type A thymoma.
Type A thymoma cells are strongly positive for AE1-defined cytokeratins. Ck20 is negative and CD20 positive cells may be detected focally. There is no expression of CD5 and BCL-2, CD57 and EMA are variable. The few lymphocytes, if present, are positive for CD3, CD5, CD1a and CD99. Immature T cells represent a minority of the T cells. CD20 B cells are usually absent.
The overall survival of patients with type A thymoma is reported at 100% at 5 and 10 years and generally, type A thymoma is regarded as a benign tumour without having a risk of recurrence if the tumour can be completely surgically resected. Exceptional case reports of local recurrence, distant metastasis or malignant transformation have been described.
Reference: WHO Classification of Tumours. Tumours of the Lung, Pleura, Thymus and Heart. 2004.