Cystic atrioventricular node tumour: not a mesothelioma.

Cystic tumours of the atrioventricular node (sometimes called ‘‘benign mesotheliomas of the atrioventricular node’’) are rare lesions associated with sudden cardiac death. 2 A recent case provided several informative points regarding the nature of this lesion and best practice methodology within the coroner’s necropsy. A 31 year old man, training for a marathon, collapsed and died suddenly without previous medical history. No system, including the heart, revealed pathology on macroscopic examination at necropsy. Toxicology and non-cardiac tissue histology were negative. However, tissue sampling of the cardiac conduction system revealed an 11 mm diameter cystic tumour of the atrioventricular node that blended into the nodal tissue and proximal His bundle (fig 1). Immunohistochemistry showed no reactivity with standard mesothelial markers (calretinin, thrombomodulin, and Wilms’ tumour 1), although reactivity for pan-keratin (AE1AE3), cytokeratin 5/6 (CK5/6), cytokeratin 7, epithelial membrane antigen, BerEp4, and carcinoembryonic antigen was noted. Mucin histochemistry, and staining for p53, Bcl-2, cyclin D1, CK20, and thyroid transcription factor 1 was negative. Mib-1 (Ki-67) staining showed minimal proliferation (2%). Previously reported immunohistology studies showed no reactivity with HBME-1 and antibody to factor VIII receptor antigen, and positive staining for secretory component and CA19.9. 4 The immunohistochemistry profile appears to support epithelial differentiation, and designation of this tumour as a form of endodermal heterotopia. This probably reflects embryonic developmental factors, with differentiation towards an upper foregut phenotype. Certainly, these results provide confirmation that the lesion should not be regarded as mesothelial. Furthermore, immunohistochemistry could also argue against a