Unusual Appearances of Uterine Leiomyomas: MR Imaging Findings and Their Histopathologic Backgrounds

Typical appearances of uterine leiomyoma at magnetic resonance (MR) imaging are well established, and diagnosis is usually easy. However, cases that are extremely difficult to differentiate from other conditions are occasionally encountered. To understand the wide spectrum of MR imaging findings, such unusual appearances can be classified into three categories: degeneration and other histopathologic findings, specific types of unusual leiomyomas, and unusual growth patterns. The common types of degeneration are hyaline (>60% of cases), cystic (∼4%), myxoid, and red. Edema is not a phenomenon of degeneration but is a common histopathologic finding (∼50% of cases). Hemorrhage, necrosis, and calcification (∼4% of cases) may also be observed. Specific types of unusual leiomyomas include lipoleiomyoma and myxoid leiomyoma, which may have MR imaging features characteristic enough to allow differentiation from other gynecologic and nongynecologic diseases. Intravenous leiomyomatosis, metastasizing leiomyoma, diffuse leiomyomatosis, and peritoneal disseminated leiomyomatosis represent unusual growth patterns; other unusual growth patterns are retroperitoneal growth, parasitic growth, and the pattern that may occur in cervical leiomyoma. Because leiomyomas are the most common gynecologic tumors and are exclusively benign, it is important to be familiar with the variety of MR imaging appearances of uterine leiomyomas to distinguish them from other significant diseases.