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Classification of thymoma by fine needle aspiration biopsy according to WHO classification: a cytological algorithm
Han Suk Ryu1, Jae Soo Koh, Sunhoo Park
1Department of Pathology, Chung-Ang University Medical Center, Chung-Ang University College of Medicine, Seoul, South Korea.
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.
