Magnetic resonance imaging characteristics of polypoid endometriosis and review of the literature

Abstract

Aim

Polypoid endometriosis is a rare variant of endometriosis and may mimic malignancy. The purpose of this study is to evaluate magnetic resonance (MR) imaging characteristics of polypoid endometriosis for the differential diagnosis with malignancy.

Methods

MR imaging findings of four histologically proven polypoid endometriosis were retrospectively evaluated with the review of the literature.

Results

All polypoid endometriosis exhibited high signal intensity on T2‐weighted images reflecting abundant dilated endometrial glands. Peritoneal lesions were surrounded by low signal intensity rim represented the “black rim sign” reflecting endometriotic fibrous adhesion. Two cases arising from endometriotic cysts showed transmural extension (peritoneal extension and myometrial infiltration). Endometriotic hemorrhagic foci were demonstrated in four lesions as high signal intensity on T1‐weighted images and/or susceptibility‐induced signal voids on susceptibility‐weighted MR sequence. Diffusion‐weighted images showed high signal intensity with relatively high apparent diffusion coefficient (ADC) due to T2 shine‐through effect but no diffusion restriction, and dynamic contrast‐enhanced (DCE) MR imaging showed gradually increasing contrast‐enhancement pattern like benign pathologies.

Conclusions

Polypoid endometriosis may mimic malignancy; however, black rim sign may be a characteristic MR imaging finding for the peritoneal lesions, and no diffusion restriction and gradually increasing contrast‐enhancement pattern may reflect its benign nature.

Introduction

Polypoid endometriosis is a rare variant of benign endometriosis with histological features resembling endometrial polyps and it was first described by Mostoufizadeh and Scully. ¹ Parker et al. reviewed the clinicopathological features of 24 cases in 2004, and broadened the definition of polypoid endometriosis to include cases of histologically usual endometriosis with polypoid growth. ² Stewart et al. divided polypoid endometriosis into two groups in 2016 as group 1: cases showed similar histological features to usual endometriosis, but involved anatomical sites that facilitated exophytic or polypoid growth; group 2: cases resembled uterine endometrial polyps histologically (fit the original definition of Mostoufizadeh and Scully), and occurred in older patients. ³ Polypoid endometriosis may typically affect peri‐ or postmenopausal women, and hormonal factors such as tamoxifen treatment can play a role in its pathogenesis. ² , ³ Actually, nearly 50% of the patients with polypoid endometriosis were on hormonal treatment at the time of diagnosis. ⁴ Because polypoid endometriosis may form multiple polypoid nodules in the peritoneal cavity or endometriotic cysts mimicking malignant tumors on imaging examination, at surgery, and on gross pathologic examination, preoperative diagnosis on magnetic resonance (MR) imaging is important to avoid aggressive treatment. ¹ , ² , ³ The literature on polypoid endometriosis focusing on MR imaging findings is limited to case reports. ⁴ , ⁵ , ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ Several MRI findings have been reported to be useful in the diagnosis of polypoid endometriosis. Takeuchi et al. reported that high signal intensity peritoneal masses with the presence of surrounding fibrous tissue showing low signal intensity on T2‐weighted images may be a diagnostic clue. ⁶ Reported cases of polypoid endometriosis referring to diffusion‐weighted imaging (DWI) showed no diffusion restriction suggesting its usefulness in differentiating from malignancy. ⁴ , ¹¹ , ¹⁶ , ¹⁸ , ¹⁹ , ²⁰ In addition, although previous reports have not mentioned their usefulness, several sequences may contribute to the MRI diagnosis of polypoid endometriosis. Dynamic‐contrast enhanced MR imaging (DCE‐MRI) is a well‐established technique for evaluating angiogenesis in malignant tumors and has been reported to be useful in differentiating between benign and malignant ovarian tumors. ²¹ Endometriotic lesions may contain blood products due to the cyclic bleeding, and detecting the blood products within the lesion is a suggestive MR finding for its endometriotic nature. Susceptibility‐weighted MR sequences (SWS) such as susceptibility‐weighted imaging (SWI) or susceptibility‐weighted angiography (SWAN) maximize sensitivity to susceptibility effects and has exquisite sensitivity to blood products. SWS was reported as a sensitive MRI technique which demonstrates hemorrhage in patients with extra‐ovarian endometriosis, and which may improve the characterization of extra‐ovarian endometriosis. ²² The purpose of this study is to evaluate the MR imaging characteristics of polypoid endometriosis including DWI, DCE‐MRI, and SWS that favor this specific diagnosis and hints for the differential diagnosis with malignancy.

Methods

Patients

The institutional review board approved this retrospective study, and waived the requirement for written informed consent. Five cases with histologically proven polypoid endometriosis had undergone MR examinations from 2005 to 2018. One case with malignant transformation was excluded and a total of four women with a mean age of 38 years (range, 30–47 years) were included in the study (Table 1). Histological diagnosis of polypoid endometriosis was made by surgical resection according to the definition of Mostoufizadeh and Scully. ¹

TABLE 1.

Cases summary

No.	Age	Site	Symptoms/hormonal state	Tumor markers
1	47	Douglas' pouch to the uterus	Lower abdominal pain associated with genital bleeding	CA125: 89 (U/mL)
2	30	Endometriotic cyst to the uterus	Lower abdominal fullness dysmenorrhea	CA125: 287 (U/mL)
3	33	Endometriotic cyst to Douglas' pouch	Menorrhagia dysmenorrhea	CA125: 251 (U/mL)
4	41	Adenomyotic cyst	Dysmenorrhea Withdrawal of a GnRH agonist	CA125: 495 (U/mL) CA19‐9: 609 (U/mL)

Abbreviations: CA125, cancer antigen 125 (normal cut‐off value: ≤35 U/mL); CA19‐9, cancer antigen 19‐9 (normal cut‐off value: ≤37 U/mL).

MR imaging

Four patients were studied on 1.5 T (one case) or 3 T (three cases) superconducting MR units based on the following parameters: (a) axial spin‐echo or gradient‐echo T1‐weighted images with/without fat‐saturation, and (b) axial and sagittal fast spin‐echo T2‐weighted images were obtained in all four cases. Scan parameters of T1‐ and T2‐weighted images varied because this study was conducted for more than 10 years. (c) MR imaging with contrast enhancement was performed with intravenous administration of gadolinium contrast agents in all four cases. In three of four cases, (d) DCE‐MRI studies were performed using three‐dimensional fast spoiled gradient‐recalled echo sequence with fat suppression with intravenous administration of 0.1 mmol/kg of gadolinium contrast agent at 2 mL/s. Images were acquired at multiple phases of contrast medium enhancement in axial or sagittal plane (pre‐contrast and post‐contrast at 30, 50, 70, 90, and 110 s) (e) Axial single‐shot echo‐planar DWI was obtained in three patients. The b factors were 0, and 800 or 1000 s/mm². (f) SWS (susceptibility‐weighted angiography: SWAN) was obtained in two patients.

Data analysis

Two radiologists with over 20 years of experiences in pelvic MR imaging qualitatively evaluated the images: signal intensity on T1‐weighted images (nonenhanced/contrast‐enhanced), T2‐weighted images, and DWI compared with that of the myometrium; the presence of low signal intensity rim as “black rim sign” which we named based on the findings reported by Takeuchi et al., ⁶ co‐existing adenomyosis (diffuse or focal thickening of the junctional zone forming ill‐defined low signal intensity areas), and pelvic endometriosis (retractile adhesions as spiculated low signal intensity peritoneal strands) on T2‐weighted images; the presence of hemorrhagic foci on T1‐weighted images as high signal intensity areas and on SWAN as susceptibility‐induced signal voids; the shape of the time‐intensity curve pattern on DCE‐MRI (type 1: gradual increase in the signal of solid tissue, without a well‐defined shoulder; type 2: moderate initial rise in the signal of solid tissue relative to that of myometrium, followed by a plateau; type 3: initial rise in the signal of solid tissue that was steeper than that of myometrium). ²¹ Agreement between the two radiologists was reached in consensus after careful individual evaluation. The mean apparent diffusion coefficient (ADC) values in two lesions were measured in a circular region of interest (ROI) in one representative region from ADC maps generated by using b‐values of 0 and 800 s/mm² on the workstation (AW4.2, GE healthcare). In these two cases, computed DWI (b = 2000 s/mm²) was generated on the workstation (ziostation2, ziosoft), and the signal intensity on computed DWI was compared with that on measured DWI (b = 800 s/mm²).

Results

MR imaging features of four cases (Figures 1, 2, 3, 4) are summarized in Table 2.

FIGURE 1.

Polypoid endometriosis arising from pelvic endometriosis in the Douglas' pouch (case 1). (a) Sagittal fast spin‐echo T2‐weighted image shows the uterine body with low signal intensity adenomyosis and high signal intensity polypoid masses with surrounding low signal intensity rim‐like structures (arrows) represented the black rim sign, which are continuous with spiculated low signal intensity strands due to pelvic endometriosis. (b) Sagittal post‐contrast gradient‐echo T1‐weighted image shows intense contrast enhancement similar to the adjacent uterine myometrium in both the masses and the surrounding structures (arrow). (c, d) Photomicrographs (hematoxylin and eosin stain) of resected mass show (c) abundant dilated endometrial‐type glands and stroma, and (d) peripheral endometriotic fibrosis surrounding the mass.

FIGURE 2.

Polypoid endometriosis arising from an ovarian endometriotic cyst with prominent myometrial infiltration (case 2). (a) Sagittal fast spin‐echo T2‐weighted image shows high signal intensity mass‐like wall thickening (arrow) of an ovarian endometriotic cyst (EC) with the shading sign infiltrating into the uterine myometrium. Worm‐like tumor extension with the bands of low signal intensity within the areas of myometrial invasion (arrowheads) corresponding to preserved muscular bundles of myometrium mimic low‐grade endometrial stromal sarcoma with myometrial invasion. (b–d) DCE‐MRI (b, pre‐contrast image; c, equilibrium phase image, sagittal three dimensional fast spoiled gradient‐recalled echo sequence with fat‐saturation; d, Time‐intensity curve) shows gradually increasing contrast enhancement pattern (type 1) in polypoid endometriosis (arrow) (circle 1) compared with the myometrium (circle 2). (e) Sagittal SWAN demonstrates numerous spotty signal voids within the polypoid endometriosis reflecting endometriotic hemorrhagic foci, and curved‐linear signal voids along the wall of the endometriotic cyst (arrows) reflecting hemosiderin‐laden macrophages combined with the fibrous nature of the cyst wall. (f) Loupe image of the mass‐like thickened wall (hematoxylin and eosin staining) shows abundant dilated glands.

FIGURE 3.

Polypoid endometriosis arising from an ovarian endometriotic cyst with peritoneal extension into the Douglas' pouch (case 3). (a) Axial fast spin‐echo T2‐weighted image shows a high signal intensity mural nodule (arrow) in a left ovarian endometriotic cyst with transmural extension into the Douglas' pouch. (b) Sagittal fast spin‐echo T2‐weighted image shows the uterine body with low signal intensity adenomyosis and high signal intensity polypoid nodules formed by the transmural extension with surrounding low signal intensity rim‐like structures (arrows) illustrated as the black rim sign. (c) Axial gradient‐echo T1‐weighted image shows a spotty high signal intensity hemorrhagic focus in the mural nodule (arrow). (d) The mural nodule (arrow) shows intense contrast enhancement on axial post‐contrast gradient‐echo T1‐weighted image with fat saturation. (e) Axial SWAN demonstrates susceptibility‐induced signal voids indicating hemorrhagic foci in the mural nodule (arrow). (f) Axial DWI (b = 800 s/mm²) shows the mural nodule (arrow) as high signal intensity with susceptibility artifacts due to hemorrhage, and (g) axial computed DWI (b = 2000 s/mm²) shows signal decrease in the mural nodule (arrow) in comparison with that of DWI (b = 800 s/mm²). (h) the mural nodule (arrow) shows high signal intensity on the correspondent axial ADC map image. The mean ADC value of the mural nodule is 1.66 × 10⁻³ mm²/s.

FIGURE 4.

Polypoid endometriosis arising from an adenomyotic cyst of the uterus (case 4). (a) Sagittal fast spin‐echo T2‐weighted image shows high signal intensity mural nodules (arrow) in an adenomyotic cyst containing clots (arrowhead). Sagittal gradient‐echo T1‐weighted images with fat saturation before (b) and after (c) the administration of gadolinium contrast agents show intense contrast enhancement in the mural nodules (arrow). The mural nodules contain mottled high signal intensity areas reflecting hemorrhage. (d) The mural nodules (arrow) show high signal intensity on axial DWI (b = 1000 s/mm²).

TABLE 2.

MR imaging features

		T2WI		T1WI		SWS	DWI	ADC	cDWI2000	DCE‐MRI	CE‐T1WI	Co‐existing features on T2WI
No.	Tesla	Signal	Black rim sign	Signal	High foci	Signal voids	Signal	(×10−3 mm2/s)	Signal decrease	TI curve	CE	Pelvic endometriosis	Adenomyosis
1	1.5	High	+	iso	+	ND	ND	ND	ND	ND	Intense	+	+
2	3	High	−	iso	−	+	High	1.57	+	type 1	Intense	+	+
3	3	High	(+) a	Low	+	+	High	1.66	+	type 1	Intense	+	+
4	3	High	−	Low	+	ND	High	N/A	ND	type 1	Intense	−	+

Abbreviations: cDWI₂₀₀₀, computed DWI (b = 2000 s/mm²); CE, degree of contrast enhancement; ND, not done; N/A, not available; TI curve, time‐intensity curve.

^a The black rim sign was observed around the peritoneal extension of polypoid endometriosis arising from endometriotic cyst to the Douglas' pouch.

Location and extension

A case arising from pelvic endometriosis appeared as multiple polypoid masses in the peritoneal cavity (Douglas' pouch) (Figure 1a–d). Two cases arising from ovarian endometriotic cysts with transmural extension (one showed peritoneal extension to the Douglas' pouch, and the other showed prominent myometrial infiltration) (Figures 2a–f, 3a–h), and one case arising from adenomyotic cyst (Figures 4a–d) were revealed.

Black rim sign

In one case arising from pelvic endometriosis (Figure 1a) and one case arising from an endometriotic cyst with peritoneal extension (Figure 3b), the black rim sign was observed around the polypoid masses in the peritoneal cavity. Whereas the black rim sign was not observed around the polypoid masses within the cysts (two endometriotic cysts and one adenomyotic cyst).

Co‐existing endometriosis and adenomyosis

Co‐existing pelvic endometriosis was observed in three cases as spiculated low signal intensity peritoneal strands on T2‐weighted images (Figure 1a). Adenomyosis was observed in all four cases as diffuse or focal thickening of the junctional zone forming ill‐defined low signal intensity areas on T2‐weighted images (Figure 1a).

Signal intensity on T2‐weighted images

All four polypoid endometriosis showed high signal intensity on T2‐weighted images compared with the myometrium (Figures 1a, 2a, 3a, b, 4a). Both intra‐cystic portion and areas of extra‐cystic extension of two polypoid endometriosis arising from endometriotic cysts showed high signal intensity on T2‐weighted images.

Signal intensity on T1‐weighted images and SWS characteristics

High signal intensity hemorrhagic foci on T1‐weighted images were observed in three of four cases (Figures 3c, 4b), whereas susceptibility‐induced signal voids reflecting hemorrhage were observed in all two lesions on SWAN: signal voids on SWAN were more prominent than high signal intensity on T1‐weighted images in one lesion, and hemorrhagic foci were demonstrated on SWAN but not on T1‐weighted images in the other lesion (Figures 2e, 3e).

DWI and ADC

DWI was obtained in three cases, and all lesions showed high signal intensity (Figures 3f, 4d). The ADC measurement was available in two lesions, and showed relatively high ADC values (1.57 and 1.66 × 10⁻³ mm²/s). The signal intensity on computed DWI with higher b value (b = 2000 s/mm²) was decreased compared with that on measured DWI (b = 800 s/mm²) in both lesions (Figure 3g).

Postcontrast imaging characteristics

DCE‐MRI was performed in three lesions and the time‐intensity curve was gradually increasing pattern (type 1) in all three lesions (Figure 2d). On post‐contrast T1‐weighted images, all four lesions showed intense contrast‐enhancement similar to that of the myometrium (Figures 1b, 3d, 4c). Peripheral low intensity rims on T2‐weighted images as the black rim sign also showed intense contrast‐enhancement on post‐contrast T1‐weighted images in all two lesions with the black rim sign (Figure 1b).

Discussion

In the current study, polypoid endometriosis showed high signal intensity on T2‐weighted images (4/4). Peritoneal lesions were associated with pelvic endometriosis and peripheral low intensity rims were observed as “black rim sign” (2/2). Hemorrhagic foci were observed within the polypoid masses on T1‐weighted images (3/4) or SWS (2/2). DWI showed high signal intensity (3/3) but no diffusion restriction with relatively high ADC (2/2). DCE‐MRI showed gradual increasing TIC (3/3), and polypoid masses exhibited intense contrast‐enhancement on post‐contrast T1‐weighted images (4/4). Only a few case reports referring to the MR imaging features of polypoid endometriosis have been documented. Our literature review using PubMed identified 17 cases of polypoid endometriosis with MR images in the reports were available. ⁴ , ⁵ , ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ MR imaging features of reported cases are summarized in Table. 3. All lesions showed high signal intensity on T2‐weighted images, and peripheral rim‐like low intensity was observed in most cases with peritoneal lesions. High intensity hemorrhagic foci on T1‐weighted images were described in four cases. DWI was obtained in seven cases and no diffusion restriction was noted in six cases (the other one case showed high signal intensity on DWI, but diffusion restriction could not be determined). Contrast‐enhancement pattern was referred in three cases as “gradual,” “mild progressive,” and “type 1 curve,” which may be equal to “gradually increasing pattern (type 1)” in our current study. In the current study, MRI findings were consistent with previous case reports, and the black rim sign on T2‐weighted images and no diffusion restriction on DWI were considered characteristic of this disease. The gradually increasing pattern of DCE‐MRI was suggested to be another characteristic of this disease based on the current study and a few previous case reports.

TABLE 3.

MR imaging features of previously reported cases

				T2WI		T1WI	DWI	ADC	DCE‐MRI	CE‐T1WI
Year	Author	Age	Site	Signal a	Black rim sign b	High foci	Signal a	(×10−3 mm2/s)	TI curve	CE a
2006	Kraft and Hughes 5	47	Endometriotic cyst	High
2008	Takeuchi et al. 6	44	Douglas pouch	High	+	+				Intense
2008	Ozaki et al. 7	33	Douglas pouch	High	+	+			Gradual	Intense
2008	Marugami et al. 8	45	Ureter	High						Intense
2010	Kwek et al. 9	48	Uterine cervix—ureter	High	(+)					Intense
2011	Lambrechts et al. 10	29	Bladder	High
2012	Kozawa et al. 11	36	Endometriotic cyst	High			sl. high	1.69		Intense
2014	Yamada et al. 12	29	Endometriotic cyst Peritoneal cavity	High	+					Intense
2015	Yamamoto et al. 13	37	Peritoneal cavity	High	(+)
2015	Choi et al. 14	66	Douglas pouch	High	(+)
2015	Gezer et al. 15	25	Douglas pouch	High	+					Intense
2016	Tham et al. 16	43	Vaginal fornix	High	+	+	sl. high	High	Mild progressive	Mild
2017	Iida et al. 17	44	Endometriotic cyst	High			High			Mild
2018	Lee et al. 18	32	Douglas pouch	High	+		Low			Intense
2020	Ghafoor et al. 4	60	Douglas pouch	High	+		High	1.4		Mild
2020	Ling et al. 19	20	Douglas pouch	High	(+)		sl. high	1.596
2020	Jacquot et al. 20	43	Douglas pouch	High	+	+	High	2.15	Type 1	Mild

Abbreviations: CE, degree of contrast enhancement; sl. high, slight high; TI curve, time‐intensity curve.

^a Because signal intensity and degree of contrast enhancement were expressed differently in different reports, we reevaluated the presented images according to our criteria.

^b The black rim sign was judged as positive: + if it was mentioned in the paper, and probable: (+) if it was not mentioned but was depicted in the images presented.

Although it was not stated whether the cases in the reports were in group 1 or group 2 according to the criteria by Stewart et al., most of the cases in which pathological findings were described were considered to be consistent with group 2. ³ , ⁴ , ⁶ , ⁷ , ⁸ , ⁹ , ¹¹ , ¹² , ¹⁴ , ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ Our four cases in the current study were diagnosed as polypoid endometriosis according to the initial definition of Mostoufizadeh and Scully, and considered as group 2. ¹ , ³ According to the review of 24 cases by Parker et al. 7 cases had multiple lesions and 17 cases had single lesions. The majority of their cases involved the surface of organs (colon 7, uterus 4, ovary 3, etc.) or occurred in endometriotic cysts (five cases). ² In our series, one case with peritoneal lesions involved the surface of the uterus and/or colon, and three cases occurred in endometriotic cysts (two ovarian and one adenomyotic cysts), which is consistent with their review. The most common clinical presentations in their series were pelvic mass (14 cases), polypoid vaginal mass (4 cases), and large bowel obstruction (3 cases). ²

In the current study, polypoid masses in all four cases showed high signal intensity on T2‐weighted images consistent with previous case reports possibly due to abundant dilated endometrial glands. ⁴ , ⁵ , ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ In Parker's review, 18 of 24 polypoid endometriosis (75%) was associated with typical endometriosis or endometriotic cyst. ² In our study, polypoid endometriosis also often co‐existed with pelvic endometriosis (75%) and adenomyosis (100%). Co‐existing pelvic endometriosis appeared as low intensity fibrous adhesion on T2‐weighted images in three cases. One case arising from pelvic endometriosis (case 1) and one case arising from ovarian endometriotic cyst with transmural extension into the peritoneal cavity (case 3) showed the black rim sign reflecting endometriosis‐induced fibrous adhesion surrounding the masses, which may be a characteristic finding for polypoid endometriosis. ⁴ , ⁶ , ⁷ , ⁹ , ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁸ , ¹⁹ , ²⁰

Endometriosis is an infiltrative disease, and may often invade the peritoneum or adjacent organs as deep pelvic endometriosis. Polypoid endometriosis may also show similar invasive tendency, ¹⁰ and in the current study two cases of polypoid endometriosis arising from endometriotic cyst showed transmural extension simulating malignant tumors with invasive tendency: prominent myometrial infiltration mimicking low‐grade endometrial stromal sarcoma with extensive myometrial invasion (case 2), and peritoneal extension with multinodular mass formation like advanced ovarian cancer with peritoneal carcinomatosis (case 3). In these cases, it was considered difficult to differentiate them from malignant tumors based on the form of extension.

Reported cases of polypoid endometriosis referring to DWI showed no diffusion restriction, ⁴ , ¹¹ , ¹⁶ , ¹⁸ , ¹⁹ , ²⁰ which may contribute to differentiate polypoid endometriosis from malignant tumors with diffusion restriction. In this study, all lesions with DWI showed high signal intensity with relatively high ADC values due to T2 shine‐through effect reflecting abundant dilated endometrial glands.

Gradually increasing contrast‐enhancement pattern (type 1) of polypoid endometriosis in DCE‐MRI may be suggestive for benignity, whereas malignant tumors generally tend to show rapid increasing contrast‐enhancement pattern (type 3) reflecting angiogenesis. ²¹

The presence of spotty hemorrhagic foci in polypoid endometriosis may be associated with aberrant endometrial tissue with cyclic hemorrhage, and may be suggestive for its endometriotic nature. In the current study, SWS (SWAN) revealed hemorrhagic foci with more sensitivity than T1‐weighted images, and may be helpful for the diagnosis. ²²

The clinical and imaging differential diseases of polypoid endometriosis include malignancies such as ovarian cancer and intra‐abdominal dissemination. Malignant transformation of endometriosis (clear cell carcinoma, endometrioid carcinoma, etc.) is particularly important as a differential diagnosis, because polypoid endometriosis may be frequently associated with pelvic endometriosis and endometriotic cyst. ² Although the presence of contrast‐enhanced solid portion in polypoid endometriosis, especially with an infiltrative extension, may mimic malignancy, the MR findings of high ADC on DWI and gradually increasing contrast‐enhancement pattern on DCE‐MRI may suggest benignity. ⁴ , ¹¹ , ¹⁶ , ¹⁸ , ¹⁹ , ²⁰ On the other hand, contrast‐enhanced solid portion in malignancy may typically show diffusion restriction on DWI and rapid increasing contrast‐enhancement pattern on DCE‐MRI. ²¹ In our study and previously reported cases, no case of polypoid endometriosis showed diffusion restriction on DWI or rapid increasing contrast‐enhancement pattern on DCE‐MRI, and is considered as MR imaging features which are helpful for the differentiation from malignancies. ⁴ , ⁷ , ¹¹ , ¹⁶ , ¹⁸ , ¹⁹ , ²⁰

Our study has several limitations: the retrospective nature of the study, small number of patients, and different MR machines with different magnetic fields (1.5 T and 3 T) used. All cases in our study were histopathologically considered to be group 2 polypoid endometriosis according to the criteria by Stewart et al., and we hypothesize that group 1 polypoid endometriosis may possibly present with different MRI findings. ³

In conclusion, polypoid endometriosis is a rare mass‐forming variant of benign endometriosis, which commonly affects the peritoneal cavity (surface of the uterus, ovary and large bowel) and within the endometriotic cysts. Polypoid endometriosis may appear as high signal intensity solid pelvic masses on T2‐weighted images occasionally with extension to the adjacent structures mimicking malignancy. The black rim sign observed on T2‐weighted images may be characteristic for polypoid endometriosis arising from pelvic endometriosis. The presence of hemorrhagic foci observed on T1‐weighted images or SWS may be suggestive for its endometriotic nature, and no diffusion restriction with relatively high ADC value and gradually increasing contrast‐enhancement pattern on DCE‐MRI may be suggestive for its benignity.

Author contributions

Takeuchi M. designed and coordinated the study. Takeuchi M, and Bando Y. collected the data. Takeuchi M, Matsuzaki K, and Bando Y. analyzed the data. Harada M. made administrative technical support. Takeuchi M, and Matsuzaki K. drafted the manuscript. All authors read and approved the final manuscript.

Conflict of interest

The authors declare that they have no conflicts of interest.

Supporting information

Data S1 PDF of reference 6

Data availability statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.