These malignant mesenchymal tumors are difficult to characterize microscopically. Undifferentiated sarcomas lack distinctive features (eg, architectural patterns, cytoplasmic and nuclear features, cell products). Anaplastic sarcomas have most of the following features: variations in size and shape of nuclei, nuclear hyperchromasia, striking irregularity of chromatin pattern, abnormal mitotic figures, and large numbers of mitotic figures. As such, anaplastic sarcomas are generally undifferentiated, but undifferentiated sarcomas do not have to be anaplastic. In both cases, wide excision is indicated; however, the prognosis is generally poorer for anaplastic sarcomas than for undifferentiated sarcomas.

Last full review/revision July 2011 by Alice Villalobos, DVM, DPNAP; Margaret Finlay, BVMS, MRCVS