Giant retroperitoneal myolipoma mimicking liposarcoma: report of a resected case and review of the literature

Abstract

Myolipomas are rare tumors that are often difficult to differentiate from liposarcoma. Herein, we report a case of resected giant myolipoma preoperatively diagnosed as liposarcoma. A 63-year-old woman was suspected of having a large retroperitoneal liposarcoma on October 202X. The patient was referred to our department for tumor resection and a histological diagnosis. After consultation with the urology, obstetric and gynecology, and vascular surgery departments, tumor resection was planned, including the potential resection of other organs. Intraoperative findings revealed a large, elastic, soft tumor with a smooth surface and a capsule occupying the entire abdominal cavity. The tumor was adherent to the stomach, left colon, and uterine adnexa, and no invasion was observed. The tumor was completely resected, and organ resection was not necessary. The tumor was 40 cm in diameter and 4.0 kg in weight. Pathological examination and immunostaining confirmed a diagnosis of myolipoma. The patient’s postoperative course was uneventful, and she was discharged on postoperative day 10 with no complications. Twelve months after surgery, the patient was doing well. To the best of our knowledge, we report a complete resection of the largest retroperitoneal myolipoma reported to date. Physicians should consider surgery, even for suspected large sarcomas that may be difficult to resect completely.

Introduction

Myolipoma is a rare benign tumor that occurs mainly in the retroperitoneum, pelvic cavity, inguinal region, round ligament, pericardium, and eyelids [1–4]. It is more common in women than in men and occurs during the fifth and sixth decades of life [5]. However, specific diagnostic criteria have not been established because of the rarity of this tumor. Although imaging is often used for diagnosis, it can be challenging to distinguish myolipomas from liposarcomas and other diseases [1, 6–8]. Herein, we report a case of a resected giant myolipoma that was difficult to differentiate preoperatively from liposarcoma.

Case report

During a physical examination in December 202X, abdominal ultrasonography revealed a retroperitoneal tumor in a 63-year-old woman. She was referred to our hospital for further examination and treatment. She had no relevant symptoms or medical history and had not taken any medication. Abdominal ultrasonography revealed a large mass on the left side of the abdomen (Fig. 1). Similarly, computed tomography (CT) revealed a large mass (40 cm in diameter) with a contrast nodule (Fig. 2A–C). ¹⁸F-fluorodeoxyglucose positron emission tomography (FDG-PET) revealed hyperaccumulation (maximum standard uptake value of 3.0) of the solid component (Fig. 3). Blood tests revealed a slight increase in the carbohydrate antigen 125 level (Table 1).

Fig. 1.

Abdominal ultrasonography at presentation. Large mass occupying the left side of the abdomen (arrow)

Fig. 2.

Abdominal computed tomography (CT) at presentation. A Axial section: abdominal CT showed a massive tumor on the left side of the abdomen. B Solid component within the tumor (arrow) and compression of the descending colon (arrowhead). C Coronal section: a large mass (dotted line) was seen occupying most of the abdominal cavity, and a nodule with high contrast (arrow) was observed inside the tumor

Fig. 3.

Positron emission tomography-computed tomography (PET-CT). Positron emission tomography-CT shows abnormal accumulation (maximum standard uptake value, 3.0) in the solid component (arrow)

Table 1.

Blood biochemistry and tumor marker test results at presentation

Biochemistry			Complete blood count
		Reference range			Reference range
TP	7.7 g/dL	6.6–8.1	WBC	3,180 /μL	3,300–8,600
Alb	4.3 g/dL	4.1–5.1	Hb	14.8 g/dL	11.6–14.8
CK	110 U/L	41–153	Plt	20.8 × 104 /μL	15.8–34.8
AST	23 U/L	13–30	Tumor marker
ALT	17 U/L	7–23			Reference range
LDH	178 U/L	124–222	CEA	1.7 ng/mL	< 3.4
ALP	73 U/L	38–113	CA19-9	6.0 ng/mL	< 37.0
BUN	12.1 mg/dL	8.0–20.0	AFP	5.7 U/mL	< 7.0
Cre	0.69 mg/dL	0.46–0.79	CA125	48 U/mL	< 35

AFP alpha-fetoprotein, Alb albumin, ALP alkaline phosphatase, ALT alanine aminotransferase, AST aspartate aminotransferase, BUN blood urea nitrogen, CA125 carbohydrate antigen 125, CA19-9 cancer antigen 19-9, CEA carcinoembryonic antigen, CK creatine kinase, Cre creatinine, Hb haemoglobin, LDH lactate dehydrogenase, Plt platelet, TP total protein, WBC white blood cell

Based on these findings, we could not rule out liposarcoma. Therefore, we planned to resect the retroperitoneal tumor. At the preoperative conference, it was emphasized that over surgery should be avoided as much as possible because a high rate of recurrence could be expected with liposarcoma, even after resection. We discussed the surgical procedure and decided to allow for a combined one-organ resection with the goal of complete resection. The final decision was left to intraoperative judgment, and the combined resection of two or more organs was considered acceptable if complete resection could be performed safely without inducing massive bleeding. Alternatively, we deemed it necessary to limit surgical intervention to excisional biopsy of regions where FDG-PET revealed abnormal accumulation. Since there was a possibility of invasion of the surrounding organs, we discussed the surgical protocol with the urology, gynecology, and vascular surgery departments before surgery. Physicians from these departments were on call during the surgery. Bilateral ureteral stents were inserted before surgery to prevent ureteral injury.

Surgery was performed in January 202X + 1. A large, soft, capsular mass occupied most of the abdominal cavity through the retroperitoneum. The feeding artery was branched from the left internal iliac artery (Fig. 4). Although we excluded the left colon and uterus, we found no invasion into the surrounding organs (Fig. 5A, B). The tumor was dissected from the surrounding organs and was removed from the retroperitoneum. The bilateral adnexa were also resected because of the large uterine broad mesometrial defect and risk of postoperative internal hernia. The surgery time was 6 h 35 min, and blood loss was 680 g.

Fig. 4.

Abdominal computed tomography (CT) at presentation (3D reconstruction and axial section). A 3D reconstruction. B Axial section. The tumor was fed by a branch (arrowhead) from the left internal iliac artery (arrow)

Fig. 5.

Intraoperative findings. Soft, capsular, and giant retroperitoneal tumors occupying most of the abdominal cavity are also observed. A Intraoperative photograph, B Schema

The tumor was a large lobular mass, 40 cm in diameter, with several vacuoles and mucous degeneration; it weighed 4.0 kg (Fig. 6A). The cracked surface was yellowish and contained areas of white color and mucous degeneration. The accumulation site on positron emission tomography-CT corresponded to a white, elastic, soft nodule (Fig. 6B). Histologically, the tumor was predominantly composed of mature adipocytes with mixed smooth muscle components and the presence of glandular epithelium in some areas, suggestive of “endosalpingiosis”. No cellular atypia or adipoblasts were observed (Fig. 7A, B). The nodules that showed abnormal accumulation on FDG-PET were smooth muscle-centered structures. No malignant findings, such as atypical cells, were observed (Fig. 8A). Immunohistochemically, mature fat cells were negative for human melanin black-45 (HMB-45) and murine double minute 2 (MDM2) (Fig. 8B), and cyclin-dependent kinase 4 (CDK4) (Fig. 8C). The spindle cells were positive for p16 and α-smooth muscle actin (α-SMA) (Fig. 8D), and the estrogen receptor (ER). The histological diagnosis was myolipoma.

Fig. 6.

Photographs of resected specimens. A The resected specimen had a large lobulated mass, 40 cm in diameter. B The cracked surface was yellowish and contained areas of white color, which corresponded to areas of high FDG accumulation and mucous degeneration (arrows and circles). FDG fluorodeoxyglucose

Fig. 7.

Histological features of the tumors. A, B There were mature adipocytes with the mixed smooth muscle component and tubular components, with the ciliated epithelium suggesting “endosalpingiosis.” Adipoblasts and cellular atypia were not observed (A, ×40; B ×12.5)

Fig. 8.

Histological features of FDG accumulation area. A Hematoxylin and eosin staining showed that the area corresponded to a nodule composed of smooth muscle, and no findings suggestive of malignancy were observed (× 12.5 upper, × 200 lower). B–D Immunostaining for CDK4 (B, ×12.5), MDM2 (C, ×12.5), and α-SMA (D, ×12.5). FDG fluorodeoxyglucose, CDK4 cyclin-dependent kinase 4, MDM2 murine double minute, α-SMA α-smooth muscle actin

The patient’s postoperative course was uneventful, and she was discharged on postoperative day 10 with no complications. Twelve months after surgery, the patient was doing well.

Discussion

Here, we report the case of a 63-year-old woman with a giant myolipoma that was difficult to differentiate from a liposarcoma. Myolipomas are benign tumors that arise in soft tissues [9]. They are relatively rare, with only a few published case reports available. Myolipomas typically feature a capsule comprising mature adipocytes and smooth muscle tissue. In most instances, the smooth muscle tissue is more prominent. Myolipomas differ from liposarcomas in the absence of lipoblasts or cellular atypia [9]. Although the mechanism of its pathogenesis is unknown, Lin et al. have suggested the involvement of abnormal lipid metabolism [10].

Myolipomas are generally benign lesions but should be distinguished from malignant lesions. Pathological differential diagnoses include liposarcoma, spindle cell lipoma, mammary-type myofibroblastoma, and fat-forming solitary fibrous tumors. On CT scans, a myolipoma may appear as a heterogeneous mass or a mass resembling subcutaneous fat. These findings radiologically resemble those of liposarcoma and angiomyolipoma [6–8]. Therefore, differential diagnosis can be challenging. In the present case, the adipocytic component lacked features suggestive of an atypical lipomatous tumor/well-differentiated liposarcoma (e.g., enlarged atypical nuclei, fibrous septation, and lipoblasts). Immunohistochemical staining indicated that the tumor was negative for malignant markers (MDM2 and CDK4). The differential diagnosis of myolipoma using immunohistochemical staining, including the staining methods mentioned in other reports, is summarized in Table 2 [11].

Table 2.

Differential immunohistochemical diagnosis of myolipomas

Tumor	Immunohistochemical staining
	SMA	Desmin	HMGA2	HMB-45	CDK4	MDM2	CDK2	CD34	Melan A
Present case	+			−	−	−			−
Myolipoma	+	+	+	−	−	−
Angiomyolipoma				+					+
Spindle cell lipoma	−	−						+
Well-differentiated liposarcoma				−	+	+	+
De-differentiated liposarcoma			+		+	+
Lipoleiomyosarcoma					+	+
Mammary type myofibroblastoma		+						+

SMA α-smooth muscle actin

There are 23 previously reported cases of retroperitoneal myolipoma (Table 3) [1, 12–15]. In these cases, the patients’ ages ranged from 35 to 79 years (mean, 57 years), and all were women. Including that in the present case, clinical symptoms have been described in six patients, with abdominal distention or an abdominal mass as the main symptom. The clinical symptoms of the remaining cases were not described. Radiologically, liposarcoma was diagnosed in four patients. No cases of myolipoma were reported preoperatively, and there was one undescribed case. Tumor diameters ranged from 3 to 30 cm in previously reported cases, making the tumor, in our case, the largest reported retroperitoneal myolipoma. Some tumors had a capsule, and the pathology comprised primarily smooth muscle in 13 cases and fat in 10 cases (including five duplicate cases), including the present case. In terms of prognosis, 13 cases showed no recurrence, and 3 cases led to death after surgery from postoperative complications or other causes. No follow-up data were available for any of the remaining patients.

Table 3.

Clinicopathological data of retroperitoneal myolipoma

Case	Age (years)	Sex	Presentation	Radiological findings	Radiological diagnosis	Size (cm)	Capsule	Predominant component	Endosalpingiosis	Prognosis
1	53	F	Incidental finding at TAH/BSO for endometrial adenocarcinoma	ND	ND	17 * 7 * 5	ND	ND	ND	No recurrence for 120 months
2	63	F	Incidental finding at repair of ruptured hepatic artery aneurysm	ND	ND	20* 17 * 11	ND	ND	ND	ND
3	52	F	ND	ND	ND	22 * 20 * 9	+	Smooth muscle	−	Death due to postoperative complication
4	62	F	ND	ND	ND	20 * 15 * 10	+	Smooth muscle	−	Death due to postoperative complication
5	79	F	Fever, lower abdominal pain	ND	Liposarcoma	30 * 30 * 17	+	ND	−	ND
6	56	F	Abdominal distension	ND	Liposarcoma	27 * 23 * 13	+	Smooth muscle	−	ND
7	55	F	Abdominal mass	Fat-intensity and reticulated low-intensity area (MRI)	Liposarcoma	30 * 15 * 8	+	Adipose tissue	−	ND
8	42	F	ND	ND	ND	3	ND	Myomatous and lipomatous tissue	ND	No recurrence for 6 months
9	60	F	ND	ND	ND	13	ND	Lipomatous tissue	ND	No recurrence for 1 month
10	75	F	ND	ND	ND	10.5	ND	ND	ND	No recurrence for 4 months
11	51	F	ND	ND	ND	ND	ND	ND	ND	ND
12	54	F	ND	ND	ND	9	ND	ND	ND	No recurrence for 41 months
13	57	F	ND	ND	ND	24	ND	Myomatous tissue	ND	No recurrence for 43 months
14	35	F	ND	ND	ND	5.2	ND	ND	ND	ND
15	50	F	ND	ND	ND	18	ND	Myomatous tissue	ND	No recurrence for 16 months
16	67	F	ND	ND	ND	ND	ND	Myomatous tissue	ND	ND
17	53	F	ND	ND	ND	28	ND	Myomatous and lipomatous tissue	ND	No recurrence for 2 months
18	50	F	ND	ND	ND	15	ND	Myomatous tissue	ND	No recurrence for 115 months
19	45	F	ND	ND	ND	12.5	ND	Myomatous tissue	ND	No recurrence for 106 months
20	65	F	ND	ND	ND	12	ND	Myomatous and lipomatous tissue	ND	ND
21	53	F	ND	ND	ND	30	ND	Lipomatous tissue	ND	No recurrence for 24 months
22	55	F	ND	ND	ND	15	ND	Myomatous and lipomatous tissue	ND	Death due to other causes
23	70	F	ND	ND	ND	8	ND	Myomatous and lipomatous tissue	ND	No recurrence for 3 months
Present case	63	F	Abdominal mass	A large mass with a contrast nodule (CT)	Liposarcoma	40 * 35 * 10	+	Adipose tissue	+	No recurrence for 12 months

CT computed tomography, F female, ND no date, TAH/BSO total abdominal hysterectomy and bilateral salpingo-oophorectomy

Endosalpingiosis is defined as the ectopic presence of a benign epithelium that histologically resembles fallopian tube epithelium. This is a common histological finding in gynecology [16]. Only a limited number of reports have addressed retroperitoneal tumors associated with endosalpingiosis. To date, four cases have been documented: three involving tumors associated with lymphangioleiomyomatosis and one with a leiomyoma diagnosis [17–19]. Notably, there have been no reports of complications related to myolipomas. There are three main theories regarding the etiology of endosalpingiosis [20]. The first, most widely accepted theory posits the regression of the cellular epithelium into the tubular epithelium [20, 21]. The second theory revolves around transplantation, in which the fallopian tube mucosa is inadvertently grafted onto the peritoneal surface during surgery [20]. The third theory suggests a pathogenesis of lymphatic metastases [22]. Since there was no history of abdominal surgery in this case and no lymph node structures were observed, we believe that the metaplastic change in the coelomic epithelium is the most likely explanation.

In the present case, liposarcoma could not be ruled out because of the presence of a large nodule that showed abnormal accumulation on FDG-PET/CT. Myolipomas may have increased accumulation on FDG-PET/CT because of increased glucose metabolism in the smooth muscle itself or inflammatory cell infiltrate [23]. The pathological findings in this case showed that the nodules primarily comprised smooth muscle, suggesting that increased glucose metabolism in the smooth muscle caused abnormal accumulation.

Extensive resection should be performed for liposarcoma, which is listed as a differential diagnosis for myolipoma. However, some tumors are considered unresectable because of their large size, invasion of surrounding organs, the unclear boundary with surrounding normal adipose tissue, or the presence of a “pseudocapsule,” which tends to invade outside of the capsule [24, 25]. Reports indicate the use of various treatments in such cases, including chemotherapy (e.g., gemcitabine, docetaxel, and doxorubicin), radiation therapy, which may involve heavy ion beams, and hyperthermia chemoradiation [26–29]. Although liposarcomas are often judged as unresectable, myolipomas, even if large, can be resected without complications in the surrounding organs, as in the present case. Therefore, surgical resection is advisable in such cases. We opted not to perform a biopsy because of the risk of seeding and damage to the adjacent organs. However, given the benefits of preoperative diagnosis, a biopsy should be performed before surgery [30].

In conclusion, in this report, we present a case of a large myolipoma with endosalpingiosis that clinically mimicked retroperitoneal malignancy. Our findings suggest that surgery may be a treatment option, even for large tumors that are expected to be challenging to resect if a myolipoma is considered a differential diagnosis.

Author contributions

KM reported the case and wrote the manuscript. KM, TI, MI, YN, CT, NT, TW, IY, and TF were involved in treating the patient. AN and KH were involved in the pathological diagnosis. TI and TF participated in revising the manuscript critically. All authors declare that they contributed to this article and that they have read and approved the final manuscript.

Funding

The authors declare that there was no financial support provided for this study.

Data availability

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

Declarations

Conflict of interest

The authors declare that they have no conflict of interest.

Informed consent

Written informed consent was obtained from the patient for the publication of this case report and the accompanying images.