Fine-needle aspiration cytology of a mediastinal seminoma associated with multilocular thymic cyst

Multiloculated thymic cysts are uncommon lesions that can be either acquired or associated with malignancies. This report describes the fine-needle aspiration (FNA) cytology of a mediastinal seminoma with prominent cystic change, confirmed by surgical pathology examination and ancillary studies performed on both the cytology and tissue specimens. The FNA cytology revealed clusters of malignant oval-to-polygonal-shaped cells with large oval nuclei possessing prominent nucleoli set in a pale-to-eosinophilic cytoplasm. These cells were surrounded by a dense lymphoid infiltrate along with a few noncaseating granulomas. The large malignant seminoma cells stained positive for placental alkaline phosphatase (PLAP) and negative for both low molecular and broad-spectrum cytokeratin. The differential diagnosis of malignancies associated with thymic cysts is presented. To the best of our knowledge, this is the first report of aspiration cytology of a mediastinal seminoma associated with a multilocular cyst.