Thymoma: an integrated clinicopathological and immunohistochemical study.

The clinicopathological features of 32 thymomas were reviewed and tumours were staged according to their degree of invasion. Their antigenic profiles were studied using monoclonal antibodies to cytokeratins (CAM 5.2 and DAKO-CK1), HNK-1 (Leu 7), and HLA-DR (TAL-IB5). Stage I (non-invasive) tumours were mainly of the spindle cell (SC) or predominantly lymphocytic (PL) types, whilst all the predominantly epithelial (PE) tumours were either locally invasive (stage II) or showed more extensive spread (stage III). Neoplastic epithelial cells all expressed cytokeratin, but varied in their degree of positivity. CAM 5.2 was more uniformly positive with cells at the periphery of tumour nodules and lining tubulo-cystic areas staining most strongly. DAKO-CK1 gave less uniform positivity but highlighted areas of medullary differentiation. HNK-1 was variably expressed in all tumour groups but was found more often in the invasive tumours (73 per cent stage III, 62 per cent stage II, 50 per cent stage I), particularly those of PE or mixed (M) type. In general, TAL-IB5 expression was lost in the more invasive thymomas. Focal medullary differentiation in tumours suggests a common origin for cortical and medullary epithelium, indicating that sub-division of tumours into cortical or medullary types is not valid. Immunohistochemistry may usefully complement clinical and macroscopic findings in the assessment of malignancy in thymoma.