Answer: Invasive thymoma, WHO type B3

Histology: none provided

Discussion: The lobulated mass displays characteristic septal fibrous bands extending from the surrounding fibrous capsule. Perivascular spaces – containing lymphocytes and bordered by palisading epithelial tumor cells – are also present, offering another clue to the diagnosis. The tumor is composed of polygonal medium to large epithelial cells exhibiting mild to focally moderate nuclear atypia. Only a minor intraepithelial lymphocytic component is demonstrated in this WHO B3 type tumor. The current tumor shows focal invasion into pulmonary parenchyma. Tumor implants were also found on the pericardial lining. The latter invasion into surrounding structures classifies our example as an invasive thymoma.

In the absence of frank cytologic features of malignancy, invasive behavior alone (like seen in our example) should not be equated with the diagnosis of thymic carcinoma.
In the current WHO classification scheme, thymomas are classified into five types (A, AB, B1, B2, B3) according to their ratio of lymphocytic/epithelial composition, degree of nuclear atypia and the shape of their epithelial component. A modified Masaoka’s classification is currently used for the staging of patients with thymoma.

Reference(s):
– Okumura M, Shiono H, Minami M, Inoue M, Utsumi T, Kadota Y, Sawa Y. Clinical and pathological aspects of thymic epithelial tumors. Gen Thorac Cardiovasc Surg. 2008 Jan;56(1):10-6. Epub 2008 Jan 22.
– World Health Organization Classification of Tumours: Pathology and Genetics of Tumours of the Lung, Pleura, Thymus and Heart. Edited by William D. Travis, Elizabeth Brambilla, H. Konrad Müller-Hermelink, Curtis C. Harris, 2004 IARC.