History: 35 year old female with pelvic pain. 

Endometriosis is a condition in which endometrial cells (the cells that line the inside of the uterus) are found outside of the uterus within other areas of the body. Patients typically complain of painful menstruation, irregular bleeding, and infertility. When endometrial tissue forms a localized mass, such as the image above, it is called an endometrioma. The typical appearance on ultrasound imaging is an adnexal cystic mass with diffuse low level internal echoes. Bright, echogenic foci are seen in the wall of the cyst in 35% of cases. Endometriomas are found in the ovary in about 75% of cases (sometimes referred to as a “chocolate cyst” in these cases), and they are bilateral in 50% of cases.

The differential diagnosis of this appearance on ultrasound includes endometrioma, dermoid cyst, hemorrhagic cyst, and cystic neoplasm of the ovary. Nodularity and thick septations should raise suspicion for malignant diagnoses.