Molecular pathology of sarcomas: concepts and clinical implications.

The molecular genetic changes that have been described in sarcomas over the past era have aided our understanding of their pathogenesis. The majority of sarcomas carry nonspecific genetic changes within a background of a complex karyotype. These constitute the challenges in sarcoma research for unraveling a putative multistep genetic model, such as for chondrosarcoma, and finding targets for therapeutic strategies. Approximately 15-20% of mesenchymal tumors carry a specific translocation within a relatively simple karyotype. The resulting fusion products act either as transcription factors upregulating genes responsible for tumor growth, as for instance in Ewing sarcoma, or translocate a highly active promoter in front of an oncogene driving tumor formation, as for instance in aneurysmal bone cyst. In addition, a small subset of mesenchymal tumors have specific somatic mutations driving oncogenesis. The specific genetic changes unraveled so far had great impact on the classification of bone and soft tissue tumors. In addition, these changes can assist the pathologist in the differential diagnosis of some of these entities, especially within the groups of small blue round cell tumors and spindle cell tumors, if performed in specialized centers. While a putative association between certain fusion products and outcome is still under debate, the role of predicting response of targeted therapy has been well established for KIT and PDGFRA mutations in gastrointestinal stromal tumors.