Differential Diagnosis for a 35-year-old Female with a Left Adnexal Lesion

Single Most Likely Diagnosis

• Endometrioma: The description of the lesion as a well-circumscribed, T1 faintly enhancing, and markedly T2 hypointense mass with no complex features is highly suggestive of an endometrioma, especially given its size (up to 3.5 cm) and the presence of proteinaceous fluid. Endometriomas are known for their characteristic appearance on MRI, which includes low signal intensity on T2-weighted images due to the presence of old blood.

Other Likely Diagnoses

• Complex Paraovarian Cyst: Although less likely than an endometrioma given the specific MRI characteristics, a complex paraovarian cyst containing proteinaceous fluid could present similarly. These cysts can have varied appearances on imaging but might show some enhancement and could contain debris or fluid with high protein content, making them a consideration.
• Hemorrhagic Cyst: A hemorrhagic cyst could potentially present with similar imaging characteristics, especially if it contains blood products at different stages of evolution. However, the stability of the lesion over time and its specific MRI features make this less likely.

Do Not Miss Diagnoses

• Ovarian Tumor (e.g., Malignant Neoplasm): Although the lesion's characteristics are not highly suggestive of malignancy (e.g., no complex features, no significant enhancement), it is crucial not to miss a potential ovarian tumor. The absence of complex features and significant vascularity reduces but does not eliminate this concern.
• Dermoid Cyst (Mature Cystic Teratoma): While dermoid cysts often have a more complex appearance with fat, calcifications, and sometimes fluid levels, a variant could potentially mimic the described lesion. The key feature that might suggest a dermoid is the presence of fat within the lesion, which is not mentioned here.

Rare Diagnoses

• Chocolate Cyst of the Broad Ligament: This is a rare condition where an endometriotic cyst forms in the broad ligament. While it could potentially mimic an adnexal mass, its location and specific characteristics might differ.
• Other Rare Cysts or Tumors: Including but not limited to, lymphangiomas, or other rare benign or malignant neoplasms of the ovary or paraovarian region. These would be considered based on atypical features or if common diagnoses are ruled out.