Myolipoma of soft tissue: an unusual tumor with expression of estrogen and progesterone receptors. Report of two cases and review of the literature

Abstract

Tumors composed of an admixture of smooth muscle and mature adipose tissues (myolipomas, lipoleiomyomas and fibrolipoleiomyomas) are very rare in the genital tract and are most often located in the uterus. We report here two cases of myolipoma: one arising in the left fallopian tube of a 43-year-old Belgian woman and the other occurring in the right broad ligament of a 69-year-old Belgian woman. Histologically, both tumors are composed of an admixture of entirely mature adipose tissue with interspersed regular smooth muscle cell bundles. This muscle component was strongly positive for actin, desmin and caldesmone antibodies and contained both numerous estrogen and progesterone receptors. Amazingly, the mature adipose cells also expressed these receptors. Interestingly, the myolipoma observed in the fallopian tube is until now, to the best of our knowledge, the first described in such a location. The myolipoma found in the broad ligament is the second to be reported in the literature to date. The differential diagnosis as well as the pathogenesis of this entity on the basis of its particular immunohistochemical profile are discussed. Mesenchymal tumors of adipose differentiation are the most common soft tissue neoplasms in adults. However, mixed mesenchymal neoplasms of the broad ligament and of the fallopian tube characterized by an admixture of adipose and smooth muscle tissue in varying proportions are extremely rare and the only previously described case is a leiomyoma with fatty degeneration of the broad ligament (lipoleiomyoma) (1). Myolipoma is a distinctive and rare tumor, which usually occurs within the abdominal cavity and the retroperitoneum (2). Histologically it is composed of an admixture of entirely mature adipose tissue with a variable amount of interspersed regular smooth muscle bundles. We describe two similar tumors occurring in the left fallopian tube and in the right broad ligament of two Belgian women. A 43-year-old, gravida 0, para 0 premenopausal Belgian woman was admitted to Saint Pierre Hospital in March 2002 for abdominal pain. Imaging demonstrated a left homogeneous tubaric mass of ‘fat’ density. At surgery, the left fallopian tube showed a grayish, well-encapsulated lesion measuring 4 × 3 × 2.5 cm. A left salpingectomy was performed. A 69-year-old, gravida 6, para 4 postmenopausal Belgian woman was admitted to Erasme University Hospital in December 2000, complaining of urinary incontinence and nycturia. An abdominal scanner revealed a well-circumscribed heterogeneous pelvic mass of ‘fat’ density. At surgery the right broad ligament demonstrated a large mass weighing 214 g and measuring 16 × 12 × 10 cm. The resection of the lesion, accompanied by a right salpingo-oophorectomy, was performed. In both cases, follow up was unremarkable. On gross examination, the lesion located in the left fallopian tube, well delimited by a thin fibrous capsule, had a soft consistency and had a grayish homogeneous appearance (Fig. 1). On the other hand, the mass situated in the broad ligament had a polylobulated appearance. It was well circumscribed and possessed a fibrous capsule (Fig. 2). On a cut section, this lesion was yellow-tan, had a firm consistency and was focally septated by a fibrous whitish tissue. Photograph of the 4-cm tubaric mass. Note the left fallopian tube completely surrounded by a grayish, homogeneous and well-circumscribed lesion. Image of the 16-cm pelvic mass, well delimited, polylobulated and showing a yellow-tan color. Inset: cut section of the lesion. Fibrous septa are clearly present. On light microscope examination, both masses revealed the presence of two distinct components: mature adipocytes and interspersed smooth muscle cells showing a strongly eosinophilic cytoplasm (Fig. 3). The mature adipose tissue was largely predominant over the smooth muscle tissue. Both components were homogeneously and harmoniously distributed all over the lesion. Histologic section of the lesions, demonstrating a mature adipose tissue intermingled with the smooth muscle component. Inset: microphotography showing smooth muscle cell nuclear positivity for estrogen receptors. Note that these receptors are also expressed by mature adipocytes. On immunohistochemical examination, the smooth muscle cells were strongly positive for smooth muscle actin, desmin, vimentin, caldesmone, and estrogen and progesterone receptors (percentage of positive cells 70% and 40%, and 80% and 90% for the fallopian tube and the broad ligament, respectively); nevertheless, they were negative for HMB-45 protein. The adipocytes showed marked positivity for S-100 protein and clearly expressed estrogen and progesterone receptors. These results suggested to us a diagnosis of myolipoma of the broad ligament. Tissues were fixed in 10% buffered formalin solution, following routine procedures. Paraffin-embedded tissue blocks were stained with hematoxilin and eosin techniques. In selected slides, we carried out immunohistochemical stains for desmine, vimentin, S100 protein, caldesmone, and estrogen and progesterone receptors using the avidin-biotin peroxidase complex method. Benign mesenchymal tumors constituted of smooth muscle cells and mature adipose tissue are exceedingly rare and comprise lipoleiomyomas, fibrolipoleiomyomas and myolipomas (3). Most often, the muscular component is predominant. Lipoleiomyomas usually develop in the uterine corpus (3, 4). Myolipomas mostly occur within the soft tissue in the abdominal cavity and retroperitoneum (2), but as per their first description by Meis and Enzinger in 1991, other locations have been reported such as the round ligament (3), blood vessels, pericardium (5), eyelid (6), intradural (7), intramedullary (8) and subcutaneous (9). The main differential diagnosis of myolipoma include well-differentiated liposarcoma, spindle-cell lipoma, angiomyolipoma and leiomyoma with fatty degeneration. Our patients' lesions were completely encapsulated. Besides, the adipose tissue component was entirely mature without lipoblasts or floret-like giant cells. Therefore, a well-differentiated sarcoma was formally excluded. Spindle cell lipoma does not show any positivity for smooth-muscle actin, desmin or vimentin, as it is exclusively formed of adipose tissue. Angiomyolipomas contain conspicuous arteries showing thick muscular walls and narrow lumina; on the contrary our case showed scarce thin-walled vessels which expressed no HMB-45 protein (angiomyolipoma blood vessels are characteristically positive for HMB-45 protein). Finally, we did not catalog this tumor as a leiomyoma with fatty degeneration because the two components, muscular and adipose, were homogeneously and harmoniously distributed within the mass: the adipose component was intrinsic to the lesion, it was an integral part of it and its distribution was not focal. Pathogenesis of myolipoma still remains unclear. There are two main theories, namely adipose metaplasia and a multipotential Mullerian cell origin (4). The presence of both numerous estrogen and progesterone receptors could be an argument for a Mullerian origin. Indeed, in our cases, like in many of the uterine leiomyomas, we've been able to demonstrate the presence of these receptors in the nuclei of smooth muscle cells. This positivity has also been described in a myolipoma reported in the pericardium (where, nevertheless, the Mullerian origin is not evident). Furthermore, and surprisingly, in our case the adipocytes equally showed this positivity (which apparently was not documented in the pericardium and in the rest of the cases reported to date), a fact that could eventually be in favor of a common Mullerian origin of the two components of the lesion. Ultrastructural analysis has shown the coexistence of both adipose (lipid-containing vacuoles) and smooth muscle cells features (myofilaments) in adipocytes throughout the lesion (5). In addition, one study analyzing three cases of uterine lipoleiomyomas showed a unique and common translocation of the tumoral cells (including the adipose component) (10). All these data suggest to us that both components of myolipoma, adipose and muscular, could stem from a common single multipotential cell of Mullerian origin. However, given the paucity of such lesions documented hitherto, further cases are required to establish an appropriate topographical, morphological, immunohistochemical and genetic profile of these neoplasias for a better understanding of their origin.