[Deciduosis peritonei--a case report]

The occurrence of ectopic decidua (deciduosis) has been observed most often in the ovaries, uterus, and cervix. It has been established in approximately 90% of patients who have undergone laparotomy during pregnancy. A peritoneal localization is less frequent and usually an asymptomatic incidental finding, but is occasionally associated with massive and potentially fatal hemoperitoneum, abdominal pain and dystocia. Ectopis decidua is a physiological phenomenon of pregnancy and arises from a progesterone-induced metaplasia of subserosal stromal cells.

A 32-year-old G1P1 female underwent cesarean section at 39 weeks gestation because of imminent fetal asphyxia. The laparotomy revealed a greyish-yellow soft nodule on the parietal peritoneum. On histologic examination the biopsy lesion was highly vascular and was composed of large polygonal decidualized cells with abundant eosinophilic, focally vacuolated cytoplasm. The nuclei were regular, with fine chromatin and inconspicuous nucleoli. Ectopic decidua was infiltrated by blood and surrounded with adipose tissue necrosis and mild chronic inflammation.

The differential diagnosis of peritoneal deciduosis included deciduoid mesothelioma, metastatic carcinoma (especially if decidual cells have a vacuolated cytoplasm mimicking signet-ring cells), and metastatic melanoma. Decidual cells showed immunoreactivity for vimentin and progesteron receptors and focal positivity for desmin and smooth muscle actin. The presence of cellular atypia, mitotic activity, and immunoreactivity for cytokeratin 5/6 and calretinin will assist in making the diagnosis of deciduoid mesothelioma. The diagnosis of metastatic carcinoma is assisted by the nuclear features and cytokeratin immunoreactivity. A positive immunostain for S-100 and/or HMB-45 will assist in the diagnosis of metastatic melanoma.