Answer: Myxoid liposarcoma

Histology: This tumor is characterized by a regular capillary vasculature which has a branching pattern. Tumor cells are generally round with minimal cytoplasm. The background is myxoid and occasionally coalesces to form acellular pools. There are areas of increased tumor cellularity, though large areas in which the tumor cell nuclei overlap are not identified.

Discussion: Myxoid neurofibromas are characterized by similar myxoid matrix; however, tumor cells are spindled and have wavy, crinkled nuclei. One typically sees strands of collagen within the tumor, and the tumor lacks the regular capillary vasculature of myxoid liposarcoma. Extraskeletal myxoid chondrosarcoma also has a myxoid matrix and is composed of plump cells. However, this tumor again lacks the regular capillary vasculature of myxoid liposarcoma. In addition, these tumors typically have areas of cord-like cell growth, and frequently have areas of overt hemorrhage. Like myxoid liposarcoma, extraskeletal myxoid chondrosarcoma is characterized by a specific chromosome translocation. This translocation fuses the EWS gene on chromosome 22q12 with the CHN gene on chromosome 9q22. Myxofibrosarcoma, also known as myxoid MFH, shares a prominent capillary vasculature and myxoid matrix with myxoid liposarcoma. However, the blood vessels in myxoid MFH are typically elongated and curved. Additional, the nuclei are less rounded and more pleomorphic.
Myxoid liposarcoma is the most common form of liposacoma, accounting for over 50% of cases. It typically arises in the soft tissues of the extremities or the retroperitoneum. Multifocal cases of myxoid liposarcoma have been identified. This tumor is characterized by a specific chromosome translocation, t(12;16) (q13;p11), that fuses the TLS gene on 16p11 to the CHOP gene on 12q13. The identical translocation is found in the round cell liposarcoma, which is therefore considered to be a higher grade variant of myxoid liposarcoma. Round cell liposarcoma is identified when nuclear overlap occurs with loss of the capillary vasculature in an otherwise myxoid liposarcoma. Recent studies have demonstrated that so-called multifocal myxoid liposarcomas are clonally related, as demonstrated by identical translocation breakpoints in genomic DNA. Hence, these “multifocal” lesions are, in fact, metastases to unusual soft tissue sites. The current case likely represents a metastasis from the thigh lesion.