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An endometrioma (also called “chocolate cyst”) is an ovarian cystic mass formed by ectopic endometrial tissue that cyclically bleeds and accumulates within the ovary. It is one of the commonest manifestations of pelvic endometriosis.

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Etiopathogenesis

Pathophysiology:
- Endometrial glands and stroma implant on the ovary → cyclic hemorrhage → trapped blood undergoes repeated hemolysis → formation of a cyst filled with altered blood and hemosiderin (“chocolate” fluid).
Risk factors: Nulliparity, prolonged estrogen exposure, family history of endometriosis.
Associations: Infertility, chronic pelvic pain, and endometriosis elsewhere in the pelvis.

Clinical Features

Chronic pelvic pain, dysmenorrhea, dyspareunia.
Infertility due to adhesions and altered ovarian reserve.
May be asymptomatic and detected incidentally.

Radiology

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Endometriomas are ovarian cystic manifestations of endometriosis, classically presenting as “ground-glass” cysts on US and “T1 bright with T2 shading” on MRI.

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Modality	Imaging features
US	• Classic finding: Unilocular cyst with homogeneous low-level “ground-glass” internal echoes due to old blood.
• No internal vascularity on Doppler.
• May be bilateral in 20–40%.
• Cyst wall usually smooth, without solid nodules (suspicious for malignancy if irregular wall or vascularized mural nodule).
MR	• T1WI: High signal intensity (blood products).
• T1 fat-sat: Persistent high signal (distinguishes from dermoid/teratoma which shows fat suppression).
• T2WI: “T2 shading” – low signal intensity within the cyst due to recurrent hemorrhage and high protein content.
• Helps detect coexisting deep pelvic endometriosis.
CT	Limited role; may show hyperdense cyst, but lacks specificity

Differentials

Hemorrhagic cyst: Hemorrhagic cysts typically resolve on follow-up and show reticular/fishnet pattern of internal echoes, while endometriomas persist with stable “ground-glass” appearance.
Dermoid: Both are T1 bright, but dermoid loses signal on fat-suppressed MRI, whereas endometrioma does not.

![The differential diagnosis of endometrial cysts includes: hemorrhagic functional cysts, fibrothecoma, cystic mature teratoma, cystic ovarian neoplasm and ovarian abscess.

Jan Hein van Waesberghe, Marieke Hazewinkel and Milou Busard, Radiology department of the VU University Medical Center Amsterdam, the Netherlands The Radiology Assistant : Endometriosis - MRI detection. Published November 1, 2011. https://radiologyassistant.nl/abdomen/unsorted/endometriosis-mri-detection](attachment:73d5eb56-8c2e-47ba-8301-4a598e084408:a50979769f366c_TEK-Diff-diagn-groot.jpg)

The differential diagnosis of endometrial cysts includes: hemorrhagic functional cysts, fibrothecoma, cystic mature teratoma, cystic ovarian neoplasm and ovarian abscess.