Atypical Fibroxanthoma.

Atypical fibroxanthoma is a malignant skin tumor with histologic features similar to those of undifferentiated pleomorphic sarcoma, but lacking its more aggressive behavior. The tumor is composed of pleomorphic cells with hyperchromatic nuclei and abundant cytoplasm, commonly arranged in a spindle cell pattern. Recent genetic studies have identified similarities between atypical fibroxanthoma and undifferentiated pleomorphic sarcoma, such as the presence of 9p and 13q deletions in both tumors, favoring a common histogenesis. However, the lack of K-ras and H-ras mutations in atypical fibroxanthoma compared with undifferentiated pleomorphic sarcoma could explain the difference in aggressiveness and continued separation of these entities. Exclusion of other neoplasms by histology and immunohistochemistry followed by complete surgical removal remains the standard of care.