Aims: Little is known about smooth muscle metaplasia (SMM) in ovarian endometriosis. The clinicopathological significance of SMM in ovarian endometriosis is analysed.
Methods and results: Three hundred and twenty-seven consecutive cases of ovarian endometriosis, including 58 cases associated with malignant ovarian epithelial tumour, between 1987 and 1996 were retrieved from hospital files and clinicopathologically analysed. The ages of the patients who showed SMM in ovarian endometriosis ranged from 28 to 70 years (average, 43. 8 years). SMM was observed in 58 cases (17.7%); in 51 of 269 cases without tumour (19.0%) and in seven of 58 with tumour (12.1%). The differences in the incidence of SMM in cases with and without tumour were not statistically significant. SMM was a focal, microscopic change and there was neither smooth muscle tumour nor uterus-like mass. Sixteen cases had short fascicles or stellate foci of SMM in the endometriotic stroma, an incomplete rim of SMM surrounding the endometriotic cysts was present in 32 cases, and 10 cases showed SMM in both the endometriotic stromal and incomplete rim of the endometriotic cysts. There was no significant histological difference in SMM between cases with and without malignant ovarian epithelial tumour.
Conclusions: SMM in ovarian endometriosis is not an uncommon phenomenon. SMM may be explained in two ways. First, smooth muscle originates from metaplastic endometrial stromal cells in endometriotic foci. Second, it originates from metaplastic ovarian stromal cells in the rim of endometriosis.