Myolipoma is a rare benign neoplasm, occurring most frequently in adults in the deep soft tissue of the abdomen or retroperitoneum. It is a benign tumour composed of variable amounts of benign smooth muscle fibres and mature adipose tissue with no lipoblasts, floret-like giant cells or zones of atypia.
The term atypical lipoma was proposed for a group of well-differentiated, non-metastasising liposarcomas arising in surgically amenable soft tissues and for deep-seated atypical adipocytic neoplasms that show variation in adipocytic size and atypical stromal cells but lack lipoblasts. However, these neoplasms recur repeatedly and may dedifferentiate and thereby acquire metastatic potential. [10
] Atypical lipomatous tumours can undergo dedifferentiation into highly malignant sarcoma [11
], a feature not reported in myolipomas.
The main differential diagnoses of myolipoma include well-differentiated liposarcoma, spindle-cell lipoma, angiomyolipoma, leiomyoma with fatty degeneration, lipoleiomyosarcoma, and leiomyosarcomas. Liposarcomas on CT scan show mild enhancement and appear to be poorly marginated and infiltrative. Liposarcomas contain lipoblasts or floret-like giant cells (enlarged eosinophilic cells with atypical nuclei) or zone of atypia. Spindle cell lipoma does not show any positivity for smooth-muscle actin, desmin or vimentin. Two recently described variants of spindle cell/pleomorphic lipoma are also presented: the pseudoangiomatous variant and the dendritic fibromyxolipoma, which corresponds in all likelihood to a peculiar myxoid variant of spindle cell lipoma. [12
] Angiomyolipomas contain conspicuous vessels showing thick muscular walls, are HMB-45 positive and frequently associated with tuberous sclerosis. A leiomyoma with fatty degeneration consists of both the components being heterogenously distributed within the mass with the adipose component focally distributed and not as an integral part of the lesion. Leiomyosarcomas of the retroperitoneum invading adipose tissue contain abundance of mitoses in the smooth muscle component and presence of recurrence or metastasis on follow-up.
Tumours consisting of a mixture of mature adipose and smooth muscle tissues, including those designated lipoleiomyomas, fibrolipoleiomyomas and myolipomas, are exceedingly rare. Most often the muscular component is predominant. Soft tissue myolipoma is a benign lesion, which has to be distinguished from lesions with malignant or uncertain biologic behaviour. Pathogenesis of myolipoma remains unclear. There are two main theories, namely adipose metaplasia and a multipotential Mullerian cell origin. [13
Myolipoma is formed by bundles of spindle-shaped cells with cigar shaped nuclei intermingled with multiloculated clear cells containing small eccentric nuclei. Histologically the tumour is constituted by two components: areas of mature fat tissue intermingled with more cellular areas composed of bundles of spindle shaped eosinophilic cells, reminiscent of smooth muscle cells. In the latter component, cells showing multilobulated, bizarre nuclei are also focally evident. No areas of necrosis or mitosis are found in both the components of the lesion. They are intricate mixtures of adult adipose tissue and bland smooth muscle exhibiting no cellular atypia or nuclear mitotic figures with little vascular proliferation.[5
] By immunohistochemistry, the spindle cells express smooth muscle actin and desmin and caldesmone antibodies and contain both oestrogen and progesterone receptors [14
]; the clear cells are non-reactive with the immunohistochemical panel, but fat is identified within the cytoplasm. The ultrastructural features of the spindle cells are those of a leiomyoma, while the clear cells are classified as adipocytes. Ultrastructural analysis has shown the coexistence of both adipocytes (lipid containing vacuoles) and smooth muscle cell features (myofilaments) throughout the lesion. [6
] The immunohistochemical and ultrastructural findings reveal the myofibroblastic nature of the cellular myoid component of the lesion.
None of the myolipomata reported showed recurrence or metastasis neither did our case. Thus a benign course and good prognosis are expected. Although myolipoma is very rare, pathologists should consider it in the differential diagnosis of fat-containing retroperitoneal masses.