High grade sarcoma with features of myxofibrosarcoma
The differential diagnosis of myxoid lesions
The differential diagnosis of myxoid lesions includes some benign entities-such as nodular fasciitis, myxoma, neurofibroma, neurothekeoma, and schwannoma-and malignant tumors such as myxoid liposarcoma, myxofibrosarcoma, and low-grade fibromyxoid sarcoma.
In some lesions the differential can be between a benign lesion such as nodular fasciitis and a malignant sarcoma such as low-grade fibromyxoid sarcoma. In the present case however, the tumor was unquestionably malignant and the primary differential was between myxoid liposarcoma and myxofibrosarcoma.
Myxoid liposarcoma and myxofibrosarcoma share many clinical and histological features making the differential diagnosis more difficult in some cases. Both lesions occur primarily in the soft tissues of the extremities and occur as slowly enlarging painless masses. Myxoid liposarcoma more commonly occurs in the deeper muscular tissues while myxofibrosarcoma more frequently arises in the superficial tissues but can also occur in the muscle (Fletcher 2002). Both are uncommon in patients younger than 20 years but the peak incidence of myxoid liposarcoma is in the 4th and 5th decade while myxofibrosarcoma is most common in the 6th to 8th decade but can be seen in a wide age range (Fletcher 2002).
As can be gathered from the names both myxoid liposarcoma and myxofibrosarcoma usually have a myxoid background. Both usually show a nodular growth pattern and can have variable cellularity. Myxoid liposarcoma can show areas of nearly solid growth which has previously been termed round cell sarcoma but now is considered to fall under the diagnosis of myxoid liposarcoma. In myxofibrosarcoma there can be high grade areas that also show nearly solid growth with little to no myxoid material between cells.
There are several morphologic features that can help differentiate between these two tumors. One is the presence of lipoblasts in myxoid liposarcoma but these are not always seen and there are many mimickers that can be misinterpreted as lipoblasts in non adipocytic lesions. The other differentiating feature is the vasculature. In myxoid liposarcoma the vessels form what has been called a "chicken wire" pattern while in myxofibrosarcoma the vessels are seen as larger curvilinear channels (Merck 1986, Menzel 1996, Fletcher 2002).
A useful feature to aid in differentiation of these two tumors is the presence of the t(12;16)(q13;p11) translocation resulting in fusion of DDIT3 and FUS in more than 90% of myxoid liposarcomas (Narendra 2011). In contrast, no characteristic molecular abnormality has been found in myxofibrosarcomas which usually show a highly complex karyotype (Downs-Kelly 2008, Guillou 2010, Nishio 2011).
In the case presented here the primary differential was between high grade myxofibrosarcoma and a myxoid liposarcoma with round cell sarcoma areas. The presence of curvilinear vessels and a lack of lipoblasts favored a diagnosis of myxofibrosarcoma and this was confirmed by the lack of a DDIT3 translocation as determined by FISH.
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Narendra S, Valente A, Tull J, Zhang S. DDIT3 gene break-apart as a molecular marker for diagnosis of myxoid liposarcoma--assay validation and clinical experience. Diagn Mol Pathol. 2011 Dec;20(4):218-24.
Contributed by Christopher C. Griffith, MD, PhD and Karen Schoedel, MD