Classification of thymoma by fine needle aspiration biopsy according to WHO classification: a cytological algorithm for stepwise analysis in the classification of thymoma.

OBJECTIVE

To evaluate the cytological characteristics of each type of thymoma and introduce an algorithm to classify thymoma using fine needle aspiration biopsy (FNAB).

STUDY

DESIGN

We retrospectively reviewed the cytological characteristics of 15 cases of thymoma with three thymic carcinoma (1 type A thymoma, 6 type AB thymomas and 8 type B thymomas), which were confirmed by histology. Three major and one minor cytomorphologic parameter were adopted for classification: (1) number of lymphocytes in the smear background; (2) nuclear characteristics of thymic cells; (3) lymphocytes and crush artifacts in thymic cell clusters, and (4) nuclear arrangement of thymic cells.

RESULTS

An abundant lymphocytic smear background indicated type B thymomas in 87.5% of cases, contrary to the few lymphocytes in the remaining thymic tumors excluding type B thymomas (90%). Thymic cells contained no vesicular nuclei and inconspicuous nucleoli in 85.7% of type A thymoma and type AB thymoma cases. Type AB thymomas and type B thymomas showed more prominent crush artifacts in cell clusters than type A thymoma and thymic carcinoma. Thymic cells of type B thymomas and thymic carcinoma were arranged without whirling architecture in clusters. The proposed algorithm demonstrated a predictive rate of 88.8% for thymoma classification.

CONCLUSIONS

The stepwise classification of thymoma with FNAB may be useful in patients for whom an invasive diagnosis approach is not feasible.