Metastatic adenocarcinoma involving a mesothelial/monocytic incidental cardiac excrescence (cardiac MICE).

Mesothelial/monocytic incidental cardiac excrescence (MICE) is a recently described, peculiar microscopic finding in the endocardium or pericardium. These lesions are characterized by a mixture of mesothelial cell clusters and histiocytes, aggregated by fibrin. They have been interpreted as reactive or even artifactual, and the importance of distinguishing these aggregations from metastatic carcinoma has been emphasized. We report a unique case of a MICE that was seeded by clusters of metastatic adenocarcinoma cells. The patient was a 38-year-old woman with no history of previous cardiac instrumentation who was found to have an adenocarcinoma of the right lung involving the hilus but apparently not invading the pericardium. At surgery, a small fragment of tissue was found floating in the pericardial cavity, and microscopic examination revealed a cluster of histiocytes, mesothelial cells and fibrin (components of usual, benign MICE) in which rare pleomorphic adenocarcinoma cells were scattered. Unlike the surrounding mesothelial cells and histiocytes, the pleomorphic cells stained for carcinoembryonic antigen, epithelial membrane antigen, and BerEP4 and focally produced intracellular mucin, confirming their malignant glandular nature. It is possible that the surrounding mesothelial cells, histiocytes, and fibrin were formed in response to invasion of the pericardial space by adenocarcinoma. This case indicates that not all lesions with the characteristic architecture of MICE can be dismissed as non-neoplastic without careful evaluation of both the cellular constituents and the clinical circumstances.