METASTATIC ADENOCARCINOMA INVOLVING A MESOTHELIAL MONOCYTIC INCIDENTAL CARDIAC EXCRESCENCE (CARDIAC MICE)/

Mesothelial/monocytic incidental cardiac excrescence (MICE) is a recen tly described, peculiar microscopic finding in the endocardium or peri cardium. These lesions are characterized by a mixture of mesothelial c ell clusters and histiocytes, aggregated by fibrin. They have been int erpreted as reactive or even artifactual, and the importance of distin guishing these aggregations from metastatic carcinoma has been emphasi zed. 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 w ith no history of previous cardiac instrumentation who was found to ha ve an adenocarcinoma of the right lung involving the hilus but apparen tly not invading the pericardium. At surgery, a small fragment of tiss ue was found floating in the pericardial cavity, and microscopic exami nation revealed a cluster of histocytes, mesothelial cells, and fibrin (components of usual, benign MICE) in which rare pleomorphic adenocar cinoma cells were scattered. Unlike the surrounding mesothelial cells and histiocytes, the pleomorphic cells stained for carcinoembryonic an tigen, epithelial membrane antigen, and Ber-EP4 and focally produced i ntracellular mucin, confirming their malignant glandular nature. It is possible that the surrounding mesothelial cells, histiocytes, and fib rin were formed in response to invasion of the pericardial space by ad enocarcinoma. This case indicates that not all lesions with the charac teristic architecture of MICE can be dismissed as non-neoplastic witho ut careful evaluation of both the cellular constituents and the clinic al circumstances.