CELLULAR BENIGN MESENCHYMAL TUMORS OF THE UTERUS - A COMPARATIVE MORPHOLOGIC AND IMMUNOHISTOCHEMICAL ANALYSIS OF 33 HIGHLY CELLULAR LEIOMYOMAS AND 6 ENDOMETRIAL STROMAL NODULES, 2 FREQUENTLY CONFUSED TUMORS

Thirty-three highly cellular leiomyomas of the uterus from patients 29 to 65 (mean, 46) years of age and six endometrial stromal nodules from patients 41 to 53 (mean, 46) years of age are described. The patients usually presented with irregular uterine bleeding. Twenty-eight of the leiomyomas were intramural and five, submucosal. Eleven of 18 whose consistency is known were soft, fish-flesh, or rubbery, 10 were yellow or yellow-tan; one had a prominent cystic component. The tumors ranged from 0.5 to 15 cm (mean, 4.6 cm) in maximum dimension. On microscopic examination, they were densely cellular and composed of cells that ranged from round to spindle-shaped and had scanty cytoplasm. A focal fascicular pattern was present in all of the cases. Blood vessels were typically large; thick muscular walls and focally dilated lumens were a conspicuous feature of the majority of the neoplasms. Arterioles were evident focally in most of the tumors but were prominent in only one of them. Cleft-like spaces, some representing compressed vessels, others due to internodular edema, were present in 24 of the tumors and were conspicuous in 15 of them. The neoplasms typically exhibited focal irregular extension into the adjacent myometrium, and this feature was conspicuous in 18 of them. They often merged almost imperceptibly with the adjacent myometrium. All tumors were immunoreactive for desmin and 32 of 32 for alpha-smooth muscle actin. Two of the stromal nodules were polypoid intracavitary masses, three were submucosal, and one intramural. Five were completely or predominantly solid with cysts present focally in three of them; one tumor was predominantly cystic. The solid tissue was frequently yellow. Microscopic examination disclosed a diffuse growth of closely packed small cells with scanty cytoplasm and nuclei that lacked atypia. Three neoplasms contained cells with abundant foamy cytoplasm that were immunoreactive for CD68, indicating that they were histiocytes and not neoplastic cells. All the neoplasms had a prominent component of arterioles, which in one tumor had hyalinized walls. Five tumors were entirely well circumscribed and one predominantly well circumscribed with limited extension into the adjacent myometrium. The tumors were immunohistochemically negative for desmin. Four of the tumors were focally or diffusely immunoreactive for a-smooth muscle actin and muscle actin. The highly cellular leiomyomas caused frequent problems in diagnosis on routine staining. Nine of them were submitted for consultation with the diagnosis of an endometrial stromal tumor, either low-grade endometrial stromal sarcoma or endometrial stromal nodule, and in an additional nine cases, the contributing pathologist was uncertain of the differential between leiomyoma and an endometrial stromal tumor. Features contributing to this diagnostic confusion were the dense cellularity of the tumors, their prominent vascularity, and their irregular margins. Features favoring a diagnosis of highly cellular leiomyoma over a stromal tumor included a fascicular growth pattern, vessels of large caliber with thick muscular walls (contrasting with the small caliber and thinner walls of the typical arterioles of an endometrial stromal neoplasm), merging with the adjacent myometrium, the presence of cleft-like spaces, an absence of foamy histiocytes, and immunoreactivity for desmin.