Benign multicystic peritoneal mesothelioma associated with partial spontaneous regression in an aged cow

This article describes a benign multicystic peritoneal mesothelioma presenting partial spontaneous regression in a 16-year-old Japanese Shorthorn cow, which exhibited innumerable fluid-filled cysts (<20 cm diameter) distributing throughout the serosal surface of the peritoneal cavity on postmortem examination. Each cyst was further subdivided into several daughter cysts. On histopathology, these cystic structures consisted of multiloculated irregular spaces that contained serous fluid positive for Alcian blue stain and periodic acid-Schiff (PAS) reaction and were lined by neoplastic mesothelial cells arranged in a single to multicell layers. In some places, neoplastic cells exhibited a tubular, papillary, villous-like, or peninsular growth pattern. Neoplastic cells exhibited mild anisocytosis and anisokaryosis with very low mitotic rate. Immunohistochemically, all or some of neoplastic cells were positive for cytokeratin or vimentin, respectively. Some smaller cysts represented abundant sclerotic stroma in association with fewer shrunken neoplastic cell-lined tubules enclosed by dense fibrocollagenous stroma, collagen fiber collapse, infiltrates of lymphocytes and macrophages disposing of collagen fiber debris, and many capillaries. These findings raised the likelihood that a specific process operating partial spontaneous regression might have been implicated in this tumor. The precise mechanism underlying this phenomenon remains uncertain, although the role of abundant sclerotic stroma was highlighted.