Disseminated thymoma in pleural fluid: An unusual case

Thymic epithelial tumours show characteristic cytological features on fine-needle aspiration cytology, however the cytological features of thymoma in fluid cytology are not well described. We present the case of a 77 year-old-woman with known pleural dissemination of type B2/B3 thymoma presenting with shortness of breath and orthopnoea due to a pleural effusion. Cytological evaluation of the pleural fluid showed cellular smears composed of numerous small lymphocytes with small numbers of admixed mesothelial cells. There was no epithelial component. On immunohistochemical (IHC) staining the lymphocytes were T cells which were positive for CD3. CD1a and terminal deoxynucleotide transferase (TdT) were also positive, consistent with immature lymphocytes of thymic origin. Despite the lack of an epithelial component, this case was diagnosed as suspicious for recurrent/ metastatic thymoma. This is only the second published case of thymoma identified on pleural fluid cytology, and to our knowledge the first case describing thymoma in pleural fluid with no epithelial component, a potential pitfall with the more common differential diagnosis of a reactive lymphocytic effusion.

Automated summary

We report a case of a 77-year-old woman with pleural dissemination of type B2/B3 thymoma, presenting with a pleural effusion. Cytological evaluation revealed numerous lymphocytes and admixed mesothelial cells, without an epithelial component. Immunohistochemical staining confirmed the lymphocytes as CD3-positive T cells, consistent with immature lymphocytes of thymic origin. Despite the absence of an epithelial component, the case was diagnosed as suspicious for recurrent/metastatic thymoma.