Decidualised ovarian endometrioma mimicking malignancy during pregnancy

A 24-year-old pregnant woman, G2P1A0, presented to our hospital at 33 weeks of gestation with a persistent right adnexal mass that was detected on routine ultrasound in the first trimester. She complained of moderate abdominal pain for a month, and a history of chronic pelvic pain without investigation, without any other systemic disease. Physical examination was normal. Laboratory findings showed elevated  AFP (132 ng/mL; normal <40) and CA 125 (45.3 U/mL; normal <35.0).

Regarding imaging findings, the MRI revealed a tumour with solid and cystic components in the right adnexa, likely originating from the ovary, measuring 15.0 cm. It appeared to be complex and multilocular, with thickened walls and fluid-fluid levels, some of which were proteinaceous with high T1 signal and T2 shading. Solid components were present, such as irregular projections with low T2 and heterogeneously high T2 mural nodules. No contrast media was injected.

US is the first-line imaging modality in evaluating a suspected adnexal mass. It has a high specificity for diagnosing a benign lesion in cases of simple cyst, haemorrhagic cyst, endometrioma, and dermoid. If the mass is indeterminant at US, MRI is helpful due to its ability to distinguish solid tissue from non-tissue solid components such as fat, blood, or debris.

As the normal endometrium, the ectopic one is hormonally responsive and can undergo bleeding, inflammation, fibrosis, and formation of adhesions during the menstrual cycle [1]. Generally, endometriosis undergoes involution during pregnancy, as decreased oestrogen inhibits cell proliferation with stasis. A process called decidualisation may happen, as a physiologic response to progesterone, better endometrial accommodation of gestation. Increased secretion from glandular epithelial cells, vascularisation of stroma, and presence of oedema cells are responsible for some of the imaging features, not usually seen in classic endometriosis [1–3].

Ultrasound imaging shows similar concerning features, such as wall thickening, irregular septa, papillary excrescences or mural nodules with increased vascularity on Doppler evaluation, which is present in both malignant lesions and decidualised endometriomas [1,3].

Differentiation from malignant complex cysts may be very challenging, as irregular and thickened walls and septa, mural nodules, and heterogeneous or haemorrhagic fluid are features that may be present in both conditions [1,2,4]. Looking carefully at the solid wall components, high signal intensities on T2-weighted imaging are highly indicative of decidualisation, as classic endometriomas and malignant solid lesions have low T2 signal [4]. Some micro cysts or a homogeneous covering lining may be the only and most important high T2 findings in a decidualised endometrioma [2,3].

Adnexal masses during pregnancy may be problematic. Surgeries during pregnancy have increased difficulty and could increase the risk of abortion and preterm birth; so, when an indeterminate complex cystic lesion is diagnosed during pregnancy, multidisciplinary discussion is mandatory to balance the foetal and maternal risks and degree of malignant suspicion [1,3]. If decidualised endometrioma is highly probable, expectant management is preferred. If there is no certainty, therapeutic options suggested are delaying surgery until delivery or postpartum if the mass showed no change in size over 4 weeks or lacked solid components and vascularity [5]. This management attempts to prevent unnecessary resection of benign tumours, or conservative observation of a malignant ovarian tumour.

Decidualised endometriomas generally have a favourable prognosis. However, rupture poses a significant concern.

Although decidualised endometriomas and malignant lesions are rare during pregnancy, the differentiation between the two entities is challenging, and surgery, although safe, must increase the risk of abortion. Decidualised endometriomas should always be suspected if a previous history of endometriosis is known, and mainly if the mural nodules have a high T2 signal or microcysts in a complex ovarian cyst [2,3].

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