Dissecting leiomyoma of the uterus with extrauterine extension

AIMS: Familiarity with benign uterine smooth muscle tumours with unusual growth patterns by gynaecologists and pathologists is essential in avoiding misdiagnosis and overtreatment. CASE DETAILS: An enlarged uterus in a 35-year-old woman contained numerous, ill-defined nodules with marked hydropic changes which showed an extrauterine extension. Histologically, although some nodules had an appearance of conventional leiomyoma, the predominant part of the lesion was characterized by ill-defined spindle cell nodules with marked hydropic changes and irregular dissection of the myometrium. The cellularity was variable from area to area and there were neither nuclear atypia nor mitotic figures. No coagulative tumour necrosis was observed. The extrauterine components, which were in continuity with the intrauterine nodule, were histologically identical to the intrauterine nodules except for the complication of endometriosis. Immunohistochemically, the tumour cells were positive for vimentin, desmin, alpha-smooth muscle actin and muscle actin (HHF35). The tumour was DNA diploid by flow cytometry. The patient was well and without disease 6 months after hysterectomy. CONCLUSIONS: This tumour illustrates that rare benign smooth muscle tumours can proliferate in dissecting and extrauterine growth patterns, findings that should not be confused with malignant mesenchymal tumours.