Peritoneal leiomyomatosis (leiomyomatosis peritonealis disseminata): a clinicopathologic study of 20 cases with ultrastructural observations.

Peritoneal leiomyomatosis (PL) occurs in women of reproductive age. Of 20 women with PL in this study, 10 were pregnant or immediately postpartum, and 7 were taking oral contraceptive steroids. Only three of the 20 women were not pregnant or taking exogenous hormones. All PL nodules were composed mainly of spindle cells, but decidual cells were admixed with the spindle cells in some of the nodules in six of the 10 pregnant and postpartum women. Electron microscopy was applied to the nodules from six women representing the three different hormonal backgrounds. All contained smooth muscle cells. One was composed exclusively of smooth muscle cells, while four of the six contained smooth muscle cells predominantly but were admixed with myofibroblasts or altered smooth muscle cells and fibroblasts. Decidual cells were conspicuous in the nodules examined ultrastructurally from a pregnant woman. Despite incomplete excisions, none of the 14 women with follow-up had progressive disease. Complete or partial regression of PL was verified by "second-look" laparotomy in four women, but a repeat cesarian section in a fifth woman revealed persistence (or recurrence) of PL 1.5 years after the initial diagnosis. PL probably results from an unusual predisposition, or selective sensitivity, of subperitoneal mesenchymal stem cells in some women to undergo metaplasia and differentiation to smooth muscle, fibroblasts, myofibroblasts, and decidual cells. In the majority of women, hormonal stimulation is an initiator or promoter of the process.