[The pathologic diagnosis of bone and soft tissue tumors: update]

Most pathologists are of the opinion that one of the most difficult and challenging fields is the pathologic diagnosis of bone and soft tissue tumors. The reasons are that these tumors represent an extremely heterogenous group of neoplasms, and many bone and soft tissue tumors have overlapping histologic appearances. This review provides an update on some of the most useful new diagnostic clues: procedure of the pathologic diagnosis, immunohistochemical usefulness, and advances in cytogenetic and molecular science. Immunohistochemistry plays a key role in the diagnosis of bone and soft tissue tumors, and the purpose of immunohistochemistry is simply to attempt to demonstrate a line of differentiation and detect protein-correlated molecular alterations in tumors. Many bone and soft tissue tumors are characterized by recurrent chromosomal rearrangements that produce specific chimeric gene fusions. Cytogenetic studies and molecular analysis using reverse-transcription polymerase chain reaction and fluorescence in situ hybridization methods can be used to diagnose tumors with specific chromosomal and gene rearrangements. They are also indicators of molecular-targeted therapy. The current WHO classification of bone and soft tissue tumors, published in 2013, is introduced and some revised points will be discussed. One notable point is that the term malignant fibrous histiocytoma (MFH) has disappeared. Undifferentiated/unclassified sarcoma has been newly added as a large group and is classified into five subtypes: undifferentiated round cell sarcoma, undifferentiated spindle cell sarcoma, undifferentiated pleomorphic sarcoma, undifferentiated epithelioid sarcoma, and undifferentiated sarcoma (NOS). Classic MFH has been replaced by undifferentiated pleomorphic sarcoma.