Cytology of thymomas - Emphasis on morphology and correlation with histologic subtypes

BACKGROUND. Aspirates of thymomas are distinguishable from other lesions an d fine-needle aspiration (FNA) is a proven method for investigating mediast inal masses. METHODS. Thirty-four cytology specimens of thymomas from 31 patients were e xamined. Corresponding surgical materials were available in 32 cases. Ten c ases were benign and 22 were malignant. Cytologic features of these thymoma s were correlated with various histologic classification systems and with b iologic behavior. Dual epithelial and lymphoid populations and irregular co hesive tissue fragments of varying proportions of lymphoid and epithelial c ells were characteristic of all aspirates. RESULTS. Using the Lattes-Bernatz classification, la cases predominately we re lymphocytic, 3 cases predominately were epithelial, 3 cases predominatel y were spindle, 15 cases predominately were mixed, and 1 case was a thymic carcinoma. In the Muller-Hermelink classification, 3 cases were medullary, 12 were mixed, 8 predominately were cortical, 2 were cortical, 6 were well differentiated thymic carcinoma, and 1 was a poorly differentiated thymic c arcinoma. In the majority of the cases the epithelial cells were round to o val. Spindle cells and a mixture of round to oval and spindle cells also we re observed. No cytologic feature was found to correlate significantly with any classification scheme. Necrosis was present in 5 of the 32 aspirates, most frequently in malignant tumors. Thymomas showing predominately spindle cells frequently were encapsulated. Tumors with predominantly round to ova l cells or a mixed population behaved more aggressively than those with spi ndle cells. Tumors that were well encapsulated and benign clinically tended to possess benign-appearing nuclei. Among the 22 invasive or malignant les ions, 8 exhibited moderate to marked cytologic atypia and 14 showed little or no atypia. No atypia was observed in benign tumors. CONCLUSIONS, The presence of cytologic atypia of epithelial cells map be he lpful in predicting aggressiveness. However, the absence of atypia and necr osis may not imply a benign course. Correlation with clinical and radiograp hic findings should be sought. Cancer (Cancer Cytopathol) 2000;90:24-32. (C ) 2000 American Cancer Society.