Endometriosis is the presence of ectopic endometrial glands outside of the uterus. MR imaging is particularly useful for characterizing deep infiltrating endometriosis but can also be useful in characterizing endometriomas and hematosalpinges, characterizing broad ligament deposits, assessing for endometriosis-associated malignancy, and differentiating malignancy from decidualized endometriomas. Masses and cysts with hemorrhagic or proteinaceous contents can sometimes be difficult to distinguish from endometriomas. Imaging protocols should include pre-contrast T1-weighted imaging with fat saturation, T2-weighted imaging without fat saturation, opposed- and in-phase or Dixon imaging, administration of contrast media, and subtraction imaging.

BACKGROUND: Ovarian endometrioma is a common gynecologic disease among reproductive-aged women. Pregnancy-related hormonal status may lead to changes of decidualization, which may resemble ovarian malignancies in sonographic appearance. Here we present a case of decidualized ovarian endometrioma clinically mimicking malignant transformation.
METHODS: A 37-year-old pregnant woman presented to our hospital at 25 + 5 weeks of gestation with a persistent left adnexal mass that was first detected on routine ultrasound in the first trimester. Transvaginal and transabdominal ultrasound showed a cystic mass of size 8.4 × 5.8 cm in the left ovary with abundant blood flow signals in the papillary medium echo of the capsule wall and inner wall, raising concern for malignant ovarian tumor. After a multidisciplinary discussion, the patient underwent laparoscopic left salpingo-oophorectomy. The results of the frozen section revealed decidualized endometrioma and the final histopathology confirmed endometrioma with extensive decidualization. The patient\'s postoperative recovery was uneventful and she was discharged on the 4th postoperative day.
CONCLUSIONS: Decidualized ovarian endometrioma is rare. Sonography and magnetic resonance imaging are helpful for differential diagnosis. Conservative management of expectant management and serial monitoring should be adopted if decidualized endometriosis is suspected.

Endometriosis constitutes the presence of ectopic endometrial glands and stroma outside the uterine endometrium, which is hormonally responsive and responds to pregnancy hormones as well. Decidualization is a physiologic process, where the normal endometrium readies itself for optimal accommodation of a pregnancy. A similar hormonal response can be seen with ectopic endometrium as well. As such, ovarian endometriomas and deep endometriosis implants can undergo decidualization. Overall, the progestational state of pregnancy favors an improvement in endometriosis, however, decidualization can lead to findings that can lead to increased size of endometriomas and deep infiltrative endometriosis implants, changes in imaging appearance and even complications, such as spontaneous hemoperitoneum in pregnancy. Awareness of this process can help prevent misdiagnosis of decidualized endometriomas as ovarian malignancy and recognize common imaging manifestations of hormonal effects of pregnancy on endometriosis.

Decidualized endometriomas (DEs) and seromucinous borderline tumors (SMBTs) exhibit similar MR findings including markedly hyperintense mural nodules within endometriotic cysts on T2-weighted images. The present study aimed to assess the efficacy of MR imaging for differentiating between DEs and SMBTs of the ovary.
MR images of 8 DEs and 14 SMBTs were retrospectively assessed and compared according to pathologies.
With regard to quantitative assessments of mural nodules, the number and signal intensity ratios (SIRs) on T1-weighted images were significantly greater in DEs than in SMBTs (11.0 ± 8.4 vs. 4.3 ± 4.1, p < 0.05 and 2.36 ± 0.56 vs. 1.49 ± 0.27, p < 0.01, respectively), whereas the height was significantly lower in DEs than in SMBTs (4.5 ± 1.4 mm vs. 21.9 ± 11.4 mm, p < 0.01). However, there were no significant differences between DEs and SMBTs in the SIRs on T2-weighted images, SIRs on diffusion-weighted images, and apparent diffusion coefficient values. With regard to qualitative assessments of mural nodules, the lobulated margin, pedunculated configuration, and T2 hypointense core were significantly more frequent in SMBTs than in DEs (71% vs. 0%, p < 0.01; 86% vs. 0%, p < 0.01; and 43% vs. 0%, p < 0.05, respectively).
The number, height, SIRs on T1-weighted images, lobulated margin, pedunculated configuration, and T2 hypointense core of mural nodules within endometriotic cysts were useful MR findings for differentiating DEs from SMBTs.

OBJECTIVE: To evaluate the clinical diagnostic ability of computed diffusion-weighted imaging (DWI) for differentiating decidualized endometrioma from ovarian cancer.
METHODS: Computed DWI technique was retrospectively applied to 20 mural nodules in 9 decidualized endometriomas during pregnancy and 20 ovarian cancers. Signal intensities on measured DWI with b values of 800s/mm(2) (DWI800) and on computed DWI with b values of 1500s/mm(2) (cDWI1500) were visually evaluated.
RESULTS: Mural nodules of all decidualized endometriomas showed high signal intensity on DWI800 with significantly higher ADC (2.01±0.26×10(-3)mm(2)/s) and low signal intensity on cDWI1500, whereas solid components of all ovarian cancers showed high signal intensity on both DWI800 with lower ADC (1.08±0.20×10(-3)mm(2)/s) and on cDWI1500.
CONCLUSIONS: Mural nodules in decidualized endometriomas may show high signal intensity on DWI800 due to T2 shine-through effect, and cDWI1500 can distinguish decidualized endometriomas from ovarian cancers by visual evaluation.

Clinical differentiation between decidualized endometrioma and malignant transformation still poses difficulties as both are intracystic vascularized excrescences of an endometrial cyst and exhibit similar characteristics on color-flow Doppler sonography. This is a characteristic sonographic finding associated with ovarian cancer, but MRI can provide further information about mural excrescences that can aid in their differential diagnosis; for example, the signal of decidualized endometriomas is isointense with the placenta within the uterus on all sequences and the apparent diffusion coefficient is higher than that of malignant mural nodules. Thus, MRI should be an aid in deciding whether to intervene during pregnancy. However, considering that it is not yet possible to clearly differentiate decidualized endometriomas from ovarian cancer, surgery or watchful observation may still be needed to exclude the possibility of malignancy.

OBJECTIVE: To identify the sonographic features of decidualized endometriomas in patients treated at a single institution and to determine whether sonographic findings can distinguish these lesions from malignant ovarian tumors during pregnancy.
METHODS: We conducted a retrospective cohort study that included pregnant women with a histologic diagnosis of decidualized endometrioma between January 1, 2005, and December 1, 2012, and had an ovarian cyst or mass seen preoperatively on obstetrical sonography. Sonographic characteristics of these masses were retrospectively evaluated using the International Ovarian Tumor Analysis Group definitions for adnexal masses.
RESULTS: Seventeen patients with 22 adnexal masses were included in our study. Nine of 22 lesions (41%) were classified as unilocular solid, and 14 of 22 (64%) had solid components, of which 12 of 14 (86%) had substantial blood flow. Septations were present in 8 of 22 masses (36%). Cyst sizes varied from 30 to 120 and 32 to 270 mm at the initial and follow-up scans, respectively. Eight patients had no follow-up scans and underwent surgery within 3 weeks of diagnosis. The other 9 patients (14 masses), had follow-up scans and underwent surgery from 3 to 34 weeks after their initial scans. Eight of these masses showed no notable change in size or appearance, and 1 became smaller.
CONCLUSIONS: There were no characteristic sonographic features identified to distinguish decidualized endometrioma from ovarian malignancy. However, lesions showing no change in size over 4 weeks or lacking solid components and vascularity are more likely to be benign rather than malignant and may justify delaying surgery until delivery or postpartum.