Answer: Trapped mesothelial cells

Histology: Within areas of dense fibrosis are trapped reactive appearing mesothelial cells, some with open, foamy cytoplasm. These cells are forming gland like spaces. There is no significant cytologic atypia.

Discussion: This patient’s history caused the consideration of a variety of diagnoses of these cells in what was otherwise a straightforward diagnosis. In a normal individual, the pericardial sac is lined by mesothelial cells. While rare mesotheliomas have been known to arise in the epicardium, most mesothelial cells seen in cardiac tissues are benign. In this case, the fibrosis in the pericardial space was likely secondary to the previous radiation and mesothelial cells were simply caught in the process. These mesothelial cells have become reactive in nature. Stains performed on the tissue showed strong positivity for calretinin, AE1/AE3 and negativity for CD31. This is not recurrent Hodgkin lymphoma due to the gland like structures that were seen. This is not adenocarcinoma of the esophagus due to the lack of severe atypia and the positive staining for calretinin. This is not mesothelioma due to the lack of severe atypia of the mesothelial cells. This is not angiosarcoma due to the lack of CD31 positive staining and lack of cytologic atypia

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