Nodular histiocytic hyperplasia of the endometrium

We describe the clinical and pathologic features of four cases of nodular histiocytic proliferation in the endometrium. We have been able to find only one brief reference to this lesion in the literature. The lesion in each case was a detached nodule composed of aggregates of histiocytes within a biopsy or curettage specimen. The constituent cells differed from foamy histiocytes of the endometrium in that they had either lobulated or ovoid, vesicular nuclei, distinctive cytoplasmic margins, and a moderate amount of amphophilic cytoplasm. Mitoses were frequent (up to 11 per 10 high-power fields) in one case but were absent in the remaining cases. On immunohistochemical staining, CD68 and lysozyme were strongly expressed in the cytoplasm. Neither estrogen receptor nor progesterone receptor was expressed in contrast to the background endometrium. The cells were also negative for S-100 and cytokeratin. Each patient's postcurettage course was uneventful. The cause of nodular histiocytic proliferation of the endometrium is currently unknown, although response to intracavitary debris has been suggested. The lesion should not be confused with a variety of reactive, inflammatory, or neoplastic conditions, such as xanthogranulomatous endometritis, malakoplakia, histiocytic granuloma, hormonal changes of the endometrial stroma, Langerhans' cell histiocytosis, morular metaplasia, extravillous trophoblast, or exaggerated placental site reaction.