Trophoblastic pseudotumor of the uterus: an exaggerated form of "syncytial endometritis" simulating a malignant tumor.

Twelve cases of a hitherto unrecognized pseudotumorous trophoblastic invasion of the myometrium are analyzed. Human chorionic gonadotropin (HCG) was identified by an immunoenzyme technique in the cytoplasm of the invasive cells. The lesion may be localized and only superficially invasive or deeply invasive and have a gross appearance suggesting a neoplasm in the excised uterus. In either case, this process has been confused with various types of malignant tumors, most often choriocarcinoma, from which it may be distinguished by an absence of the characteristic dimorphic population of cytotrophoblast and cyncytiotrophoblast. Most of the patients, who ranged in age from 19 to 41 years, presented with amenorrhea uterine enlargement and were thought to be pregnant, although only four of them had positive pregnancy tests. The most serious complication was uterine perforation, which occurred spontaneously in one patient and during curettage in five others. One patient died as a result, but the other 11 are alive without evidence of disase 6 months to 12 years after the diagnosis. Of the 11 surviving patients, four received no treatment other than curettage. Because this trophoblastic lesion is likely benign and simulates a malignant tumor on pathologic examination, the term "trophoblastic pseudotumor" has been chosen to designate it.