Immunohistochemical analysis of thyroid transcription factor-1 in type A and AB thymomas comparing multiple primary...

Thyroid transcription factor-1 (TTF-1) is expressed in the thyroid gland, lungs, and brain, and has also been reported in thymomas. While some studies have suggested TTF-1 as a specific marker for type AB thymoma, others have noted significant variability, often due to small case numbers. In this study, TTF-1 expression was evaluated in 50 cases of type A and AB thymomas using three distinct clones (8G7G3/1, SPT24, and SP141), and quantified with a four-grade scale. Additionally, CD20 expression and GTF2I mutation status were analyzed. TTF-1 expression analyses with the SPT24 and SP141 clones yielded a score of 3 (defined as positivity in >50 % of cells) in 20 % and 18 % of thymomas, respectively. In contrast, the 8G7G3/1 clone showed a score of 0 (positivity in <1 % of cells) across all tumors. There were no differences in score 3 positivity between type A and type AB thymomas for either the SPT24 or SP141 clones. Additionally, no statistically significant associations were identified between clinicopathological characteristics or GTF2I mutations and score 3 versus score ≤2 cases for both clones. Tumors with a CD20 score of 3 demonstrated a tendency towards higher SPT24 or SP141 expression. Different expressions by TTF-1 clone may be diagnostic challenges in type A/AB thymomas, and pathologists should be aware of aberrant TTF-1 expression using SPT24 and SP141 clone in thymomas.