Decidualized Ovarian Endometrioma in a Pregnant Woman Mimicking Ovarian Malignancy: Magnetic Resonance Imaging and Ultrasonographic Findings

Adnexal masses discovered in pregnancy are commonly benign in nature with conservative management being the norm after careful imaging evaluation. The percentage of malignant adnexal masses is less than 1%. However, even benign masses can be symptomatic and cause obstetric complications ( 1). Recommendations for conservative or surgical intervention during pregnancy are largely dependent on the clinician’s assessment of the patient. Factors such as perceived risk of ovarian cancer, gestational age at presentation as well as risk of adverse events for the mother or fetus require consideration. It is crucial to make an informed decision about the risk of malignancy, as managing suspected ovarian cancer in pregnancy needs extensive multidisciplinary expertise and cooperation of specialists in neonatology, perinatology, gynecology, oncology, and obstetric anesthesiology Fortunately, most malignant ovarian neoplasms during pregnancy are detected at an early stage with a fairly good outcome for both mother and child ( 10). In pregnancy, decidual changes of the endometrium are predominantly caused by high levels of progesterone. Ovarian endometriosis is commonly seen in pregnancies, but ovarian endometrioma decidualization is very rare ( 3). The high progestin state of pregnancy is generally thought to have beneficial effect with regression of endometriosis, however, as in this particular case, paradoxical progression with growth and rupture of the cystic lesion may occur ( 7). Although ultrasonography remains the primary method for detection and assessment of adnexal masses in pregnancy due to its ease of access and safety features, the findings are often nonspecific. These nonspecific cases in which sonography is unable to produce a definitive diagnosis of the adnexal masses usually require MRI as an adjunct ( 4). MR imaging in pregnancy is usually performed without gadolinium enhancement due to the potential risk of teratogenic effect to the fetus ( 11). In the presence of excess of endogenous and exogenous estrogens, ovarian endometriosis can undergo malignant transformation in 0.7% to 1% of cases ( 12). Endometrioma with malignant transformation during pregnancy has not been reported yet to our knowledge. Solid nodule formation within an endometrioma remains a potential concern and could indicate superimposed malignancy in all patients irrespective of pregnant status. Furthermore, as in this case, both the ultrasound and MRI signs of a decidualized ovarian endometrioma can resemble malignant tumors,which often poses a diagnostic challenge. However, prospective imaging differentiation of decidualization from malignant transformation may be feasible. Suggested imaging criteria from a few case reports have been made in the past. In sonography, the imaging finding that may help to suggest decidualization is the presence of smooth and lobulated solid nodules within an endometrioma with marked internal vascularity that are detected during early pregnancy and do not grow significantly on serial imaging. In contrast, the solid components in ovarian cancer are often irregular, found within a fluid-containing complex adnexal mass. These solid components also demonstrate increased Doppler vascularity. Increased vascularity, one of the sonographic signs of suspicious pelvic tumors, occurs in decidualized tissue due to thickening of the stroma and procurement of an extensive vascular supply. Three dimensional (3D) sonography enhances the images obtained, allowing the papillary appearance of the tumor to be clearly outlined via transvaginal 3D surface rendering, and also clear rendering of the rich vascularity through the use of 3D power Doppler imaging ( 5, 8). In MR imaging, decidualized endometriomas and decidualized endometrium of the gravid uterus share strikingly similar appearances in terms of marked high signal intensity and the overall texture. In contrast, in malignant transformation, the mural nodules on T2 weighted imaging are slightly high to intermediate intensity ( 7). In a recent article, the authors have reported utilizing diffusion-weighted MR imaging (DWI) and apparent diffusion coefficient (ADC) measurements as a diagnostic aid for this rare entity. It was reported in the article that the high signal intensity on DWI was non-specific for either decidualized endometrial tissues or malignant mural nodules. However, the ADC values of the two entities allowed differentiation as decidualized endometrial tissues had significantly higher values compared to that of malignant ovarian tumors. This manifestation is likely due to the rich cytoplasmic content of the stromal cells within the edematous vascularized decidualized endometrial tissue that leads to prolonged T2 and higher ADC values in comparison to those of malignant tumors. The restricted diffusion in malignant tumors is due to the higher cellularity and diminished extracellular space that causes suppressed water motion ( 13). The CA-125 level is elevated during pregnancy, which is a physiological occurrence. Therefore, this marker, which is normally useful in the assessment of malignant tumors, becomes limited in value during pregnancy. However, serial assessment with unchanged values may be able to predict the benignity of the lesion ( 14). Additionally, as illustrated in this case, complications due to rupture of the large endometrioma can occur. In our case, rupture of the cyst could be postulated secondary to the increase in tension on the cyst as the uterus enlarged to accommodate the growing fetus thus reducing the abdominal space and presence of dense adhesions between the ovary and the surrounding structures, compounding the tension. Other theories include the effect of increased blood flow to the ovaries during pregnancy resulting in cyst enlargement and possible directly bleeding into the cyst itself, leading to rupture. Decidualization of the stroma may also lead to softening of the cyst and eventual rupture during pregnancy ( 15). In conclusion, decidualized ovarian endometrioma in pregnancy tends to present as a complex adnexal cystic mass, which can mimic ovarian malignancy. However, awareness of its occurrence and the specific imaging findings using the combination of both ultrasonography and MR examination, a prospective diagnosis may be possible for this rare entity, thus allowing appropriate clinical management to be made.