Last revised by Daniel J Bell on 17 Mar 2024

Endometriomas, also known as chocolate cysts or endometriotic cysts, are a localized form of endometriosis and are usually within the ovary. They are readily diagnosed on ultrasound, with most demonstrating classical radiographic features. 

These occur in up to 10% of women of reproductive age.

The symptoms do not necessarily correlate with disease severity and include pelvic pain, dysmenorrhea, dyspareunia and infertility in 30-40% of patients.

Although the pathogenesis is still under discussion, three theories have emerged:

  1. metaplastic transformation of the peritoneal epithelium into functional endometrium

  2. peritoneal seeding due to retrograde menstruation

  3. activation of mesenchymal cells differentiation caused by endometrium in the peritoneal cavity from retrograde flow

Endometriomas contain dark degenerated blood products following repeated cyclical hemorrhage. The cysts may be up to 20 cm in size although they are usually smaller (2-5 cm).

Typical locations include:

Not usually helpful in diagnosis. ~10% of endometriomas can calcify.

The appearances of endometriomas can be quite variable. The classical example is an avascular unilocular cyst containing low-level, homogeneous "ground-glass" like internal echoes, as a result of the hemorrhagic debris. This appearance occurs in 50% of cases 7.

Less typical features include 7:

  • multiple locules (~85% will have <5 locules)

  • hyperechoic wall foci (present in 35%) due to cholesterol deposits 

  • cystic-solid lesion (~15%) or purely solid lesion (1%)

  • anechoic cysts (rare: 2%)

  • fluid-fluid level 10

CT is not the preferred imaging modality to assess endometriosis, as image findings tend to be non-specific, nevertheless often patients will undergo CT for acute abdominal pain.

Endometriomas on CT tend to be complex pelvic cystic masses often with increased attenuation components representing hemorrhage. These appearances are non-specific also being seen with hemorrhagic ovarian cysts or neoplasms. Bilateral lesions points towards endometriomas but again maybe seen with other entities, such as a tubo-ovarian abscess or neoplasm 10.

Signal characteristics vary according to the age of any complicating hemorrhage 6:

  • T1

    • typically, lesions appear hyperintense while acute hemorrhage occasionally appears hypointense

    • endometriomas with high T1 signal characteristically do not show loss of signal on T1 fat-suppressed sequences, which is important for differentiating it from a mature cystic teratoma of the ovary

  • T2

    • typically hypointense owing to the presence of deoxyhemoglobin and methemoglobin (shading sign), which is very suggestive of an endometrioma 3

    • T2 dark spot sign is specific for chronic hemorrhage and is helpful in diagnosing endometriomas 9

    • old hemorrhage occasionally appears hyperintense


    • variable restricted diffusion

  • T1 C+ (Gd)

    • may have wall enhancement

    • the presence of an enhancing mural nodule is suggestive of malignant transformation

Although endometriomas are usually benign entities, there is an ~1% rate of malignant transformation. Endometrioid tumors of the ovary and clear cell ovarian carcinomas are the most common histological patterns seen 8. They are mostly seen in women >40 years after several years of latency, with endometriomas larger than 9 cm 4,5. Malignant transformation is uncommon in masses <6 cm.

If not surgically excised, follow-up should be at least yearly 4. GnRH agonists may be used for medical management.

Endometriomas have the potential to decidualise during pregnancy resulting in the formation of vascularized, papillary projections 13. Although rare, these changes give an appearance mimicking malignancy. As such, expert gynecological ultrasound review is prudent in such cases.

Ovarian endometriomas carry a lower risk of ovarian torsion than other ovarian cysts due to their frequent association with adhesions 14.

General imaging differential considerations include:

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