Collagenous Spherulosis in Well-Differentiated Papillary Mesothelioma in a Female With Multilocular Peritoneal Inclusion Cysts

Multilocular peritoneal inclusion cysts are mesothelial tumor-like lesions with or without small foci of florid papillary mesothelial hyperplasia that might simulate serous epithelial papillary lesions or well-differentiated papillary mesothelioma (WDPM) of the peritoneum. The papillary cores in papillary mesothelial hyperplasia and in WDPM display a variety of different histomorphologic features. To our knowledge, collagenous spherulosis in WDPM was not previously described or illustrated in the literature. Collagen spherules have been described in several miscellaneous benign and malignant lesions of the breast, prostate, salivary gland, and cutaneous adnexal tumors. They sometimes impose a diagnostically challenging cribriform growth pattern. We report a case of a young adult woman who presented with a large painful palpable multicystic mass in the right iliac fossa and pelvis. Histologically, it showed features of multilocular peritoneal inclusion cysts with an incidental finding of a small nodule demonstrating morphologic and immunohistochemical features of WDPM. The papillary cores revealed uniform homogenous eosinophilic round collagenous spheres with a cribriform pattern. The unusual finding of collagen spherules may expand the morphologic spectrum of mesothelial papillary lesions, the differential diagnosis of female pelvic peritoneal and omental epithelial and nonepithelial lesions and might cause some diagnostic confusion on small specimens. It may also shed some light on the pathophysiology and role of mesothelial and submesothelial stromal cells in the variable morphologic patterns of the stroma of papillary cores in different mesothelial lesions.