Decidualized Endometrioma During Pregnancy: An O-RADS Pitfall

Essence of the Phenomenon

Decidualization of endometrioid foci during pregnancy leads to the appearance of papillary structures within the endometrioma, which on ultrasound examination may be suspicious for malignant transformation. This process has been described both for ovarian endometriomas and for extragonadal endometriosis foci (e.g., bladder involvement), mimicking neoplastic processes (Lambrechts S, et al.; Chertin B, et al.).

Ultrasound Findings

According to series describing ovarian masses with papillary projections diagnosed and removed during pregnancy, decidualized endometrioma is characterized by papillary projections that may be accompanied by vascularization and create sonographic features suspicious for malignancy (Fruscella E, et al.; Mascilini F, et al.). Features more suggestive of benign decidualization are smooth contour of papillary projections and characteristic "ground glass" (low-level) cyst contents: according to series, smooth contour of papillary projections (≈79% versus 27%) and ground-glass echogenicity (≈74% versus 13%) in benign masses are more frequent than in malignant lesions (Mascilini F et al., 2014). Papillary projections in decidualization are typically vascularized on color or power Doppler; therefore, the mere presence of blood flow does NOT exclude a benign process—herein lies the pitfall. Features supporting decidualization include known prepregnancy endometrioma, smooth or lobulated solid nodules, and stability on follow-up ultrasound (absence of significant growth), whereas in malignant processes the solid component more often has an irregular contour. Because of substantial morphologic overlap, definitive differentiation often requires consideration of clinical history, dynamic changes, and, when necessary, MRI.

Clinical Significance and O-RADS Pitfall

The main problem is excessive suspicion and unjustified surgical removal of masses during pregnancy. Studies including clinical and ultrasound characteristics of decidualized endometriomas surgically removed during pregnancy emphasize the discrepancy between alarming sonographic appearance and benign histology (Mascilini F, et al., Ultrasound Obstet Gynecol 2014;44:354-360).

Natural Evolution During Pregnancy

The morphology of deep endometriosis and ovarian endometrioma may change during pregnancy, as described in ultrasound studies of the natural history of endometriosis in pregnancy (Bean E, et al., Ultrasound Obstet Gynecol 2023;62:585-593). Consideration of this dynamic evolution is important when interpreting changed imaging findings in a pregnant patient with known endometrioma.