Problematic zones in the classification of soft tissue tumors

: The classification of soft tissue tumors is based on the recognition of the resemblance to normal tissue or cells. Nowadays, molecular pathologic findings essentially may contribute to the diagnosis. In daily practice, however, the evaluation of HE sections and immunohistochemical findings are most important because these methods are widely available. Nevertheless, misinterpretations are possible if certain rules and limitations in data utilization for diagnosis are not considered. There are some problematic zones referring to this in which especial attention is mandatory. By means of examples difficulties are explained which may result from overlapping morphological features between soft tissue tumors, between nerve sheath tumors and melanocytic neoplasms, and between soft tissue tumors and sarcomatoid carcinomas. The necessity of a careful interpretation of immunohistochemical findings is underscored with selecting actin positivity as example. Finally, difficulties in determining the dignity of a soft tissue tumor are discussed. Moreover, tumor heterogeneity may under certain conditions render more difficult the classification of a soft tissue tumor.