Florid soft-tissue decidual reaction: a potential mimic of neoplasia

While undergoing a repeat Caesarian section, a 21-year-old woman was found to have a subcutaneous, 3.5-cm tumor-like lesion in the abdominal wall. It had a lobulated contour and consisted of gelatinous tan nodules with intervening fibrous septa. It had a composite histology, including a solid pattern of large glassy cells with a predominant perivascular and pericystic/cisternal topography (35% surface area); a pattern of dyscohesive, vacuolated cells including physaliphorous-like forms (35% surface area); and a myxoid pattern of spindle to stellate cells showing frequent cell contact and lying within an optically clear or pale eosinophilic and bubbly extracellular matrix with a prominent capillary framework. A strong and diffuse cytoplasmic expression of vimentin (only) was present in all cytoarchitectural patterns. The extracellular matrix and intracytoplasmic vacuoles contained abundant acid mucosubstance, mostly hyaluronic acid. No distinct endometrial gland, stigma of hemorrhage, or nuclear estrogen/progesterone receptor protein expression was observed. There were variously sized and shaped cystic spaces lined by a flat to cuboidal cytokeratin-positive cell lining and focally containing neutral and acid mucosubstance. These spaces are interpreted as dilated endometrial glands rather than mechanically entrapped inclusions of mesothelial origin. In this setting, florid decidual reaction represents a potential diagnostic pitfall because it could be confused with more commonly encountered myxoid or epithelioid tumors of mesenchymal, epithelial, or melanocytic cell lineage.