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Endometriomas,also known as chocolate cysts or endometriotic cysts, are a localised 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.

Clinical presentation

The symptoms do not necessarily correlate with disease severity and include pelvic pain, dysmenorrhoea, 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 haemorrhage. The cysts may be up to 20 cm in size although they are usually smaller (2-5 cm).


Typical locations include:

Radiographic features

Plain radiograph

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


The appearances of endometriomas can be quite variable. The classical example is a unilocular cyst with acoustic enhancement with diffuse homogeneous ground-glass echoes as a result of the haemorrhagic 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%)
  • cystic-solid lesion (~15%) or purely solid lesion (1%)
  • anechoic cysts (rare; 2%)
  • fluid-fluid level 10

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

  • T1
    • typically, lesions appear hyperintense while acute haemorrhage occasionally appears hypointense
    • endometriomas with high T1 signal characteristically do not show loss of signal on T1 fat-suppressed sequence, which is important for differentiating it from mature cystic teratoma of the ovary
  • T2
    • typically hypointense owing to the presence of deoxyhaemoglobin and methaemoglobin (shading sign), which is very suggestive of endometrioma 3
    • T2 dark spot sign is specific for chronic haemorrhage and is helpful in diagnosing endometriomas 9
    • old haemorrhage occasionally appears hyperintense
  • DWI
    • variable restricted diffusion
  • T1C+
    • may have wall enhancement
    • the presence of an enhancing mural nodule is suggestive of malignant transformation

Treatment and prognosis

Although endometriomas are usually a benign entity, there is an ~1% rate of malignant transformation. Endometrioid tumours of the ovary and clear cell adenocarcinoma are the most common histological pattern 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.

Differential diagnosis

General imaging differential considerations include:

  • +<li>fluid-fluid level <sup>10</sup>
  • +</li>

References changed:

  • 10. Bennett G, Slywotzky C, Cantera M, Hecht E. Unusual Manifestations and Complications of Endometriosis—Spectrum of Imaging Findings:Pictorial Review. AJR Am J Roentgenol. 2010;194(6_supplement):WS34-46. <a href="https://doi.org/10.2214/ajr.07.7142">doi:10.2214/ajr.07.7142</a> - <a href="https://www.ncbi.nlm.nih.gov/pubmed/20489127">Pubmed</a>

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Cases and figures

  • Case 1
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  • Case 2: endometrioma, fibroid and ovarian cyst
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  • Case 3
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  • Case 4
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  • Case 5
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  • Case 6
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  • Case 7: chocolate cyst in right adnexa
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  •  Case 8
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  • Case 9: T1 C+ fat sat
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  • Case 10: MRI T1
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  • Case 11
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  • Case 12
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  • Case 13
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  • Case 14
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  • Case 15: bilateral ovarian
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  • Case 16
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  • Case 17
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  • Case 18
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  • Case 19: MRI signs
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  • Case 20
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  • Case 21: shading sign
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  • Case 22: ruptured endometrioma
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  • Case 23
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  • Case 24
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  • Case 25
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