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Deciduoid mesothelial hyperplasia of the pelvic peritoneum

Sir: It is well recognized that the histological distinction between hyperplastic mesothelial cells and malignant mesothelioma or metastatic carcinoma can be problematic. Reactive mesothelial proliferations can form disorganized multilayered sheets and tubulopapillary structures, and, perhaps most worryingly, mesothelial cells may become entrapped within reactive stroma, mimicking invasion. Furthermore, benign mesothelial cells may be identified within vascular channels or lymph nodes, with the obvious potential for misdiagnosis. I have encountered a further pattern of reactive mesothelial proliferation involving the pelvic peritoneum of three patients, characterized by morphological changes reminiscent of decidualized endometrial stroma, thus potentially mimicking the deciduoid variant of malignant mesothelioma (DMM). To my knowledge, similar changes have been noted only once previously. The patients were aged 59, 53 and 35 years, and presented with ovarian adenosquamous carcinoma, ovarian serous cystadenoma, and tubal ectopic pregnancy, respectively. Concurrent hysterectomy, contralateral salpingo-oophorectomy, omental biopsy and peritoneal fluid cytology specimens in the first two cases showed benign findings. The third case showed organizing serosal haemorrhage, and immature erythroid and granulocytic precursors consistent with extramedullary haematopoiesis (EMH) were present within the associated granulation tissue. This patient was mildly anaemic preoperatively, but had no known significant haematological disorder. In each case, there was a focal deciduoid histological alteration characterized by a multilayered, sheet or cord-like proliferation of mesothelial cells showing enlarged vesicular nuclei, distinct nucleoli, and abundant, predominantly eosinophilic, ‘glassy’ cytoplasm, often with distinct cell borders, thus resembling decidualized endometrial stromal cells (Figures 1 and 2). Immunohistochemistry was positive for cytokeratin (CK)7, CK5/6, calretinin, desmin, mesothelin, D2-40, and WT1, at least focally, in all cases. The mesothelial cells were arranged at the peritoneal surface in cases 2 and 3, where focal continuity with ‘conventional’ reactive mesothelium was apparent (Figure 1). A