Cytology of thymomas: emphasis on morphology and correlation with histologic subtypes

Aspirates of thymomas are distinguishable from other lesions and fine-needle aspiration (FNA) is a proven method for investigating mediastinal masses.

Thirty-four cytology specimens of thymomas from 31 patients were examined. Corresponding surgical materials were available in 32 cases. Ten cases were benign and 22 were malignant. Cytologic features of these thymomas were correlated with various histologic classification systems and with biologic behavior. Dual epithelial and lymphoid populations and irregular cohesive tissue fragments of varying proportions of lymphoid and epithelial cells were characteristic of all aspirates.

Using the Lattes-Bernatz classification, 10 cases predominately were lymphocytic, 3 cases predominately were epithelial, 3 cases predominately 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 carcinoma. In the majority of the cases the epithelial cells were round to oval. Spindle cells and a mixture of round to oval and spindle cells also were 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 oval cells or a mixed population behaved more aggressively than those with spindle cells. Tumors that were well encapsulated and benign clinically tended to possess benign-appearing nuclei. Among the 22 invasive or malignant lesions, 8 exhibited moderate to marked cytologic atypia and 14 showed little or no atypia. No atypia was observed in benign tumors.

The presence of cytologic atypia of epithelial cells may be helpful in predicting aggressiveness. However, the absence of atypia and necrosis may not imply a benign course. Correlation with clinical and radiographic findings should be sought.