Peritoneal myolipoma

Clinical History

A 39-year-old woman without pre-existing condition presented with acute abdominal pain on the left iliac fossa for the past 48 hours, without fever. She also reported abdominal distension during the last 4 months. Physical examination revealed abdominal dullness to percussion.

Imaging Findings

Abdominal ultrasound showed a large abdominopelvic solid heterogeneous mass. CT was performed to better characterize the lesion, demonstrating a smooth-contoured, encapsulated, predominantly cystic-appearing (homogeneous water attenuation) mass containing multiple areas of fat attenuation and faint enhancing nodular solid soft tissue components (Figs. 1-3). The tumour measured 20 x 16 x 19 cm and was located in the abdominopelvic cavity, without clear relationship with any organ. The patient underwent surgery.

The histopathological study revealed grossly a spherical and firm mass with soft yellow areas on the cut surface with a whorled trabecular pattern (Fig. 4). Microscopically the tumour was composed of an intermixture of mature fat cells, smooth muscle without atypia and collagen. The tumour cells showed immunoreactivity for smooth muscle markers (desmin, actin and calponin) and were negative for melanocitic markers (MELAN A and HMB-45) (Fig. 5). The diagnosis was consistent with lipoleiomyoma.

Discussion

Myolipoma of soft tissue, also known as lipoleiomyoma, is an extremely infrequent benign lipomatous lesion. It was first described as a separate entity in 1991 by Meis and Enzinger [1] and only a few cases have been reported since then. This lesion occurs most frequently in adults with a female predilection [2].

Myolipomas are most frequently located in the retroperitoneum, abdominal cavity, pelvis, abdominal wall and inguinal areas. Rare cases have been reported in other locations, such as the subcutaneous tissue of the extremities [2].

Symptomatology depends on the location and size of the lesion. Typically, myolipomas are very large at diagnosis (mean 15 cm), particularly when they are discovered in the abdominal cavity or in a retroperitoneal location [3]. Patients usually present with abdominal distension or a soft-tissue mass. Less frequently, the lesions are asymptomatic and incidentally discovered. Subcutaneous lesions are detected earlier because of their superficial location, so they are much smaller at diagnosis [2].

Myolipoma is constituted by two components: smooth muscle and mature adult adipose tissue. Macroscopically, these tumours are circumscribed and usually encapsulated with a white to yellow appearance [2], depending on the relative proportions of muscle and fat. Studied by immunohistochemistry, smooth muscle components are positive for smooth muscle markers (desmin, actin, calponin) [2]. No zones of atypia or mitoses are present and the tumour shows little vascular proliferation [4, 5].

The imaging findings reflect the mixed histopathological composition of myolipomas, with both adipose components and nonadipose solid components representing smooth muscle. Adipose components have specific imaging features of fat tissue. The nonadipose components demonstrate nonspecific solid features and appear as ill-defined areas with soft-tissue attenuation on CT images, intermediate signal intensity on T1-weighted MR images and intermediate to high signal intensity on T2-weighted MR images [2, 6, 7]. There is no significant contrast enhancement. There are no pathologically detectable lymph nodes [3]. Coarse calcifications have also been reported [2, 6, 7].

Myolipomas may be radiologically misdiagnosed as liposarcoma, particularly if they are located in the retroperitoneum [5, 8]. Histopathological assessment is necessary because it is not possible to distinguish between this entity and other lipomatous lesions on the basis of imaging characteristics alone.

The treatment of choice for these lesions is surgical excision [5]. To the best of our knowledge, there is no evidence of metastatic disease, malignant transformation or local recurrence in the available literature [2].

Differential Diagnosis List

Final Diagnosis

Myolipoma of soft tissue or lipoleiomyoma

URL:	https://www.eurorad.org/case/13506
DOI:	10.1594/EURORAD/CASE.13506
ISSN:	1563-4086