Thymoma : a clinicopathologic association of world health organization histologic subtype and invasive behaviour

To re-classify thymic epithelial neoplasms reported at Aga Khan University Hospital during the past seven years according to the revised WHO classification, to assess the ease of application and determine association between WHO histological subtype and invasive behaviour.

Descriptive study.

The study was carried out in the section of Histopathology, Department of Pathology and Microbiology, Aga Khan University Hospital, Karachi, from January 2000 to October 2006.

All cases of thymic epithelial neoplasms reported in the past seven years were retrieved using SNOMED coding system. Small biopsies where the tissue was insufficient for definite classification were not included. All cases were reviewed and reclassified according to WHO classification into types A, AB, B1, B2 and B3. Capsular invasion as well as extension into neighboring structures such as perithymic fat, pleura, pericardium, lung etc. was noted on morphology.

A total of 62 cases were diagnosed as Thymic Epithelial Tumors (TET). Out of these, there were 5 type A (6 %), 17 type AB (21%), 7 type B1 (8.6%), 26 type B2 (32%) and 7 type B3 (8.6%) thymomas. Age range was from 22-78 years with a median age of 46 years. Male to female ratio was 6:1. History of associated myasthenia gravis was present in 21% of cases. A significant association was observed between WHO histologic subtype and invasive behaviour where types A, AB and B1 have lesser number of invasive cases as compared to non-invasive, whereas in types B2 and B3, more cases have shown invasion as compared to non-invasive cases (c2 = 14.093, df =1, p-value < 0.001 ).

The WHO classification is simple and easy to apply and has significant association with aggressive behavior. To some extent, it reflects the clinical behaviour of thymomas along with stage and status of resection. However, morphologically benign looking thymomas can behave aggressively. Hence, tumour stage, extent of resection and histology should be combined to predict the clinical behaviour of thymomas.