Once a pelvic mass is identified on an ultrasound examination, the first step in the differential diagnostic work up is to determine its origin. Most lateral pelvic masses in women are ovarian in origin, and the distinction between ovarian and nonovarian mimics of ovarian cancer is critical for appropriate clinical and surgical management. Adnexal masses detected on ultrasound can be further characterized by magnetic resonance imaging (MRI) when needed. Superior contrast resolution, multiplanar imaging, characteristic signal intensity of common pathology such as dermoid tumors or endometriomas allows one to accurately evaluate adnexal tumors with supplemental use of MRI. Commonly encountered extraovarian abnormalities that mimic ovarian malignancies are categorized as being either predominantly cystic or solid. The common causes of such extraovarian lesions that mimic ovarian pathology include fallopian tube diseases, paroaovarian cysts, peritoneal inclusion cysts, and a pedunculated or a broad ligament fibroid. Less common causes of cystic and solid nonovarian mimics of ovarian malignancy include mucocele of the appendix, lymphocele, spinal meningeal cysts, extraovarian endometriomas, extraovarian fibrothecomas, and gastrointestinal stromal tumors (Table 1). Identifying a normal appearing ovary is the key in distinguishing an extraovarian pelvic mass from an ovarian tumor. This becomes particularly challenging in postmenopausal women with atrophic ovaries. In this scenario, MRI comes into use by identifying small atrophic ovaries more often than ultrasound is able to. Extraovarian lesions typically displace the pelvic sidewall vasculature medially, ureters tend to be compressed, encased or medially displaced, enhancement matches pelvic arteries and may be associated with engorged mesenteric vessels compared to gonadal vessel engorgement seen with ovarian tumors.
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