Pathology of Leiomyomas

Leiomyomas of the uterus have been stated to be present in 20 to 30% of women over the age of 30 years. In one study of total hysterectomy specimens in which gross serial sectioning at 2 mm intervals was done, however, leiomyomas were found in 77 of 100 cases; they were detected as frequently in women whose uteri had been removed for clinical indications other than leiomyomas as in those with a preoperative diagnosis of leiomyomas; in 84% of the specimens the leiomyomas were multiple. Leiomyomas are designated submucous if they lie in contact with the endometrium. Typically, as submucous leiomyomas enlarge, they bulge into the uterine cavity, forming sessile or pedunculated polyps, which may prolapse through the cervical os; ulceration and hemorrhage are common complications. Occasional "submucous" leiomyomas may arise within the endometrium instead of the myometrium because endometrial stromal cells have the capacity to differentiate into smooth muscle cells. Subserosal leiomyomas often become pedunculated and may twist on their pedicles, with resultant hemorrhagic infarction; also, distended veins on the surface of a subserosal leiomyoma may rupture, causing massive intraperitoneal hemorrhage. Some subserosal leiomyomas become detached from the uterus and reattached to adjacent organs or tissues and are designated "parasitic leiomyomas. " Most leiomyomas are intramural, occupying the midportion of the myometrium. Sectioning of a leiomyoma typically reveals bulging tissue that is softer than the surrounding myometrium and has a whorled appearance with white, fibrous trabeculae coursing around tan or gray-pink softer tissue (Fig. 1). Typically, the tumor is sharply demarcated from the surrounding myometrium, from which it can be easily enucleated, but occasionally the margin of the tumor blends almost imperceptibly with the adjacent muscle. Microscopic examination of the typical leiomyoma discloses intersecting fascicles of elongated spindle cells with central, typically pale, cigar-shaped nuclei and elongated bands of eosinophilic cytoplasm emanating from each nuclear pole. Rarely, nuclear palisading similar to that seen in neurilemomas is a prominent feature. The smooth muscle cells are separated by parallel reticulin fibrils and varying amounts of collagen. The tumor may appear relatively avascular or contain numerous vessels, including large muscular arteries, arterioles, and thinwalled venous channels. Tumor often protrudes into dilated veins as polypoid projections. The margin of the tumor is typically sharp, but occasionally there is limited focal extension of the tumor into the adjoining myometrium. A variety of "degenerative" changes may take place in leiomyomas, altering their microscopic and often their gross appearance. Hyaline change is characterized by homogenization of the collagen between groups of tumor cells and individual tumor cells, with the hyaline material occasionally replacing large portions of the neoplasm. A more striking alteration in a leiomyoma is edema, or hydropic degeneration, which is often associated with hyalinization. When the edema is diffuse, numerous thinwalled cysts filled with watery fluid may form (Fig. 2), resulting in a large, boggy tumor that may be confused with a leiomyosarcoma at operation. At times, the hydropic degeneration is perinodular, surrounding small lobules of tumor (Fig. 3). After fixation and staining, the hypocellular, watery tissue may become inapparent artifactually, creating a pat-