Answer: Angiosarcoma

Histology: This is an extremely difficult case without immunohistochemistry, particularly in view of the young age of the patient. The section shows fibrotic pericardium with an area showing spindle cell features similar to a reactive mesothelial proliferation. However, in amongst the connective tissue is an endothelial proliferation showing anastomosing channels, hobnail endothelium, and occasional mitoses. On immunohistochemical staining, the proliferating cells were reactive with CD34 but lacked cytokeratin and calretinin. Given the infiltrative nature and large size of this lesion, it was diagnosed as angiosarcoma.

Discussion: Such tumors sometimes present in a lung-encasing fashion reminiscent of mesothelioma and with the application of the appropriate immunohistochemical panel it is difficult to distinguish these lesions. Of course, a mesothelioma would be expected to express epithelial markers and calretinin. In the present example, epithelial markers were negative but it is well known that epithelioid angiosarcomas can express epithelial markers and therefore a panel approach to immunohistochemistry is always advised. As a further word of caution, mesothelial cells, both reactive and neoplastic, can express desmin, which can cause diagnostic confusion should the patient be evaluated for skeletal muscle tumors. The prognosis for this patient is poor.