Pleomorphic lipoma, considered as a variant of spindle cell lipoma, typically presents in older men with a median age of more than 55 year. The majority of the lesions occurs in the subcutis of the posterior neck, back, and shoulder area
. Less frequently, it can occur in palm
, tonsillar fossa
 and oral cavity
. Our reported case occurred in axilla, which was also a relatively uncommon location. Pleomorphic lipomas are benign tumors, can be readily treated by excision, although occasionally recurrences can happen.
Histologically, pelomorphic lipoma is largely composed of mature fat and bland spindle mesenchymal cells. In addition, multinucleated giant cells are scattered amid the spindle cells, and their nuclei are radically arranged in a “floret-like” pattern. The spectrum of histology shows a wide variation, and varies from tumor that resembles ordinary lipoma with few spindle cells to tumor that mainly consists of spindle cells with just a few fat cells. Cytologically, spindle cells have single elongated hyperchromatic nuclei and inconspicuous nucleoli, whereas multinucleated giant cells have irregular, hyperchromatic and significantly atypical nuclei. The mitoses of the two cell types are rare. “Ropelike” collagen bands are randomly distributed amid the cellular elements, usually an important diagnosis clue to pelomorphic lipoma. Some lesions may have extensive myxoid stroma, which can be a dominant feature and pose a diagnostic challenge. Some inflammatory cells including mast cells, lymphocytes and plasma cells are often scattered among the spindle cells. The vascular pattern usually consists of a few small or intermediate-sized, thick-walled vessels. In contrast, some examples have a prominent plexiform vascular pattern reminiscent of myxoid liposarcoma
. A pseudoangiomatous variant of pleomorphic lipoma has also been described
. Immunohistochemically, the spindle cells and “floret-like” cells are strongly positive for CD34, but negative for S-100 protein and smooth muscle actin
[4, 5]. Some cases can show desmin positive expression, which may lead to a misdiagnosis of a smooth muscle tumor
The differential diagnosis of classic pleomorphic lipoma includes atypical lipomatous tumor/well-differentiated liposarcoma and pleomorphic liposarcoma. The typical pleomorphic lipoma usually arises in the subcutaneous tissue of shoulder or head and neck region. But, atypical lipomatous tumor/well-differentiated liposarcoma and pleomorphic lipoma usually arises in deep soft tissue of extremities or retroperitoneum. “Floret-like” giant cells in pleomorphic lipoma occasionally can also be seen in atypical lipomatous tumor/well-differentiated liposarcoma, which is not suitable for distinguishing them. The ropey collagen can not be seen in atypical lipomatous tumor/well-differentiated liposarcoma and pleomorphic liposarcoma, so it is useful for differential diagnosis. Moreover, pleomorphic lipoma lacks lipoblasts, which can be seen in the above two lesions
However, if a tumor with the background of pleomorphic lipoma abosolutely lacks the fat cells, whether the tumor still should be diagnosed as a pleomorphic lipoma? In fact, it is debated by some pathologists. In our reported case, spindle cells and “floret-like” cells are scattered in the extensive myxoid stroma and the adipose tissue was absolutely absent. It poses a great diagnostic challenge. So far, there is only one literature that described two cases of pleomorphic lipoma with devoid of mature fat tissue. That is, Sachdeva MP et al. reported a review of 38 pleomorphic lipomas seen in consultation revealed 2 cases in which fat was absent. It was called the fat-free variant of pleomorphic lipoma
. They believed that diagnosis of pleomorphic lipoma mainly depended on the context of the tumor and nonlipomatous components rather than lipomatous components. In our case, because of the absence of fat, we first thought that it might be a myxoid fibrosarcoma. Myxoid fibrosarcoma is more common than pleomorphic lipoma in elderly male. The majority of them occur in extremities, rarely on the trunk or head and neck area. Histologically, myxoid fibrosarcoma usually consists of spindle cells and multinucleate giant cells, and is characterized by prominent elongated, curvilinear, thin-walled blood vessels. The spectrum of myxoid fibrosarcoma is also variable. However, myxoid fibrosarcoma tends to have more cellular atypia and mitosis activity. Moreover, myxoid fibrosarcoma usually lacks strong CD34 expression
. So, myxoid fibrosarcoma can be ruled out.
In addition, the differential diagnosis also includes giant cell fibroblastoma, pleomorphic fibroma and giant cell angiofibroma, which also show CD34 positive expression and possess multinucleated giant cells. Giant cell fibroblastoma is a juvenile form of dermatofibrosarcoma protuberans (DFSP). It affects predominantly infants and children. Its characterized feature is that multinucleated tumor cells often line slitlike pseudovascular spaces
. Pleomorphic fibroma is characterized by hypocellular dermal proliferations of spindle and irregularly shaped stellate or multinucleate cells with storiform and clefted hyalinized collagen
. Giant cell angiofibroma is a giant-cell-rich form of solitary fibrous tumor. It displays all the features of a classic SFT but is identified by pseudovascular spaces lined by multinucleated stromal giant cell
. Occasionally, multinucleated giant cells could also be seen in neurofibroma
[23, 24], which show CD34 and S-100 positive expression. Although our case lacks mature fat tissue, according to the morphological and immunohistochemical findings, at last, the tumor was diagnosed as a pelomprphic lipoma.